drm/i915/reg_defs: Add REG_FIELD_MAX wrapper for FIELD_MAX()

drm/i915/reg_defs: Add REG_FIELD_MAX wrapper for FIELD_MAX()

Introduce REG_FIELD_MAX macro as local wrapper around FIELD_MAX() to return
the maximum value representable by a bit mask. The value is cast to u32
for consistency with other REG_* macros and assumes the bitfield fits
within 32 bits.

v2: Use __mask as macro argument aligning with other macros. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250924141542.3122126-8-ankit.k.nautiyal@intel.com