drm/amd/display: Add regamma lut write mask to SOC base

Mask and shift values for DCP0_REGAMMA_LUT_WRITE_EN_MASK were missing
from XFM_COMMON_MASK_SH_LIST_SOC_BASE. Add it.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>