[Why]
According to HW team, PG is dropped for NV12, but programming
the registers will still cause power to be consumed, so don't
program for NV12.
[How]
Set function pointer to NULL if NV12
Signed-off-by: Alvin Lee <alvin.lee2@amd.com> Reviewed-by: Jun Lei <Jun.Lei@amd.com> Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com> Acked-by: Harry Wentland <harry.wentland@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>