drm/dp: Add smooth brightness register bit definition

Add DP_EDP_SMOOTH_BRIGHTNESS register bit definition for
EDP_GENERAL_CAPABILITY 2 register.

--v2
-Add eDP 2.0 comment [Ankit]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://lore.kernel.org/r/20250411060235.2732060-2-suraj.kandpal@intel.com