drm/amd/display: Add interface for ADD & DROP PIXEL Registers

[WHY]
HW has handed down a new sequence that requires access to these
registers.

v2: squash in DCN3.1 fixes (Alex)

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>