drm/amd/display: Fix overlay with pre-blend color processing

[WHY]
Overlay works similarly to MPO, but uses global alpha on both planes and
sets the desktop as the rear plane instead of the front plane

[HOW]
Ensure that top plane isn't overlay by checking global alpha before
applying the previously added MPO fix

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>