drm/amd/display: Fixes for some MPO cases

[Why]
Alpha could be improperly applied (only affecting half the
frame) for some source pixel formats.

[How]
Change how alpha is enabled in MPC/DPP LB and change the
bottom plane blend mode in MPC.

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@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>