drm/amd/display: Remove flag check in mpcc update

[Why]
MPCC programming was being missed during certain split pipe enables due
to full_update flag not being true. This caused a momentary flash on
half the screen. After discussion, determined we should not have that
flag check within update_mpcc, as it should always perform full
programming when called.

[How]
Remove flag check. We call update_blending within insert_plane, so we
do not need to replace its call from the if block.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>