drm/amd/display: introduce mpo detection flags

[why]
We want to know if new crtc state is enabling MPO configuration before
enabling it.
[how]
Detect if both primary and overlay planes are enabled on the same CRTC.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>