drm/i915/dram: Move 16Gb DIMM detection fully to the skl/icl codepaths

We are incorrectly applying the 16Gb DIMM w/a (adding 1 extra
usec to WM0 latency) on MTL+ even though the w/a is only needed
for SKL/ICL.

The current way of setting this is up is a bit of a disaster:
1. always set has_16gb_dimms=true for all platforms except BXT/GLK
2. has_16gb_dimms potentially gets overwritten with something else
 * BXT/GLK don't do anything since we never set has_16gb_dimms to
   begin with
 * skl_get_dram_info() overwrites has_16gb_dimms with the actual
   detection results for SKL/ICL/derivatives
 * gen12_get_dram_info() (correctly) resets has_16gb_dimms
   for TGL/ADL/derivatives
 * xelpdp_get_dram_info() doesn't do anything, leaving
   has_16gb_dimms incorrectly set for MTL+

Clean up the whole mess by only setting has_16gb_dimms in the
SKL/ICL codepaths where we have the actual detection code for
it. This avois applying the w/a incorrectly on MTL+.

v2: Rewrite commit msg (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250902133113.18778-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

Author: vsyrjala

drivers/gpu/drm/i915/soc/intel_dram.c

@@ -406,6 +406,9 @@ skl_dram_get_channels_info(struct drm_i915_private *i915, struct dram_info *dram
 	u32 val;
 	int ret;
 
+	/* Assume 16Gb DIMMs are present until proven otherwise */
+	dram_info->has_16gb_dimms = true;
+
 	val = intel_uncore_read(&i915->uncore,
 				SKL_MAD_DIMM_CH0_0_0_0_MCHBAR_MCMAIN);
 	ret = skl_dram_get_channel_info(i915, &ch0, 0, val);
@@ -435,6 +438,9 @@ skl_dram_get_channels_info(struct drm_i915_private *i915, struct dram_info *dram
 	drm_dbg_kms(&i915->drm, "Memory configuration is symmetric? %s\n",
 		    str_yes_no(dram_info->symmetric_memory));
 
+	drm_dbg_kms(&i915->drm, "16Gb DIMMs: %s\n",
+		    str_yes_no(dram_info->has_16gb_dimms));
+
 	return 0;
 }
 
@@ -673,8 +679,6 @@ static int gen11_get_dram_info(struct drm_i915_private *i915, struct dram_info *
 
 static int gen12_get_dram_info(struct drm_i915_private *i915, struct dram_info *dram_info)
 {
-	dram_info->has_16gb_dimms = false;
-
 	return icl_pcode_read_mem_global_info(i915, dram_info);
 }
 
@@ -736,12 +740,6 @@ int intel_dram_detect(struct drm_i915_private *i915)
 
 	i915->dram_info = dram_info;
 
-	/*
-	 * Assume 16Gb DIMMs are present until proven
-	 * otherwise, this w/a is not needed by bxt/glk.
-	 */
-	dram_info->has_16gb_dimms = !IS_BROXTON(i915) && !IS_GEMINILAKE(i915);
-
 	if (DISPLAY_VER(display) >= 14)
 		ret = xelpdp_get_dram_info(i915, dram_info);
 	else if (GRAPHICS_VER(i915) >= 12)
@@ -766,9 +764,6 @@ int intel_dram_detect(struct drm_i915_private *i915)
 
 	drm_dbg_kms(&i915->drm, "DRAM channels: %u\n", dram_info->num_channels);
 
-	drm_dbg_kms(&i915->drm, "16Gb DIMMs: %s\n",
-		    str_yes_no(dram_info->has_16gb_dimms));
-
 	return 0;
 }