drm/amd/display: Add reg defs for DCN303

[WHY]
These registers are currently missing from the DCN303 header files

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>