drm/tidss: dispc: Explicitly include bitfield.h

After a recent series to use FIELD_PREP and FIELD_MODIFY in
tidss_dispc.c, there are many errors when bitfield.h is not implicitly
included, such as when building allmodconfig for ARCH=hexagon:

  drivers/gpu/drm/tidss/tidss_dispc.c:1116:2: error: call to undeclared function 'FIELD_MODIFY'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   1116 |         VP_REG_FLD_MOD(dispc, hw_videoport, DISPC_VP_CONTROL, v,
        |         ^
  drivers/gpu/drm/tidss/tidss_dispc.c:631:3: note: expanded from macro 'VP_REG_FLD_MOD'
    631 |                 FIELD_MODIFY((mask), &_reg, (val));                     \
        |                 ^
  drivers/gpu/drm/tidss/tidss_dispc.c:1140:2: error: call to undeclared function 'FIELD_MODIFY'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   1140 |         FIELD_MODIFY(DISPC_VP_DSS_OLDI_CFG_MAP_MASK, &oldi_cfg,
        |         ^
  drivers/gpu/drm/tidss/tidss_dispc.c:1203:10: error: call to undeclared function 'FIELD_PREP'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   1203 |                        FIELD_PREP(DISPC_VP_TIMING_H_SYNC_PULSE_MASK, hsw - 1) |
        |                        ^
  ...

Explicitly include bitfield.h to resolve the errors.

Fixes: 9accc8b ("drm/tidss: dispc: Get rid of FLD_VAL")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250902-drm-tidss-fix-missing-bitfield-h-v1-1-aaad4a285f98@kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Author: nathanchance

drivers/gpu/drm/tidss/tidss_dispc.c

@@ -4,6 +4,7 @@
  * Author: Jyri Sarha <jsarha@ti.com>
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/delay.h>
 #include <linux/dma-mapping.h>