drm/amd/display: Reject overlay plane configurations in multi-display scenarios

[Why]
These aren't stable on some platform configurations when driving
multiple displays, especially on higher resolution.

In particular the delay in asserting p-state and validating from
x86 outweights any power or performance benefit from the hardware
composition.

Under some configurations this will manifest itself as extreme stutter
or unresponsiveness especially when combined with cursor movement.

[How]
Disable these for now. Exposing overlays to userspace doesn't guarantee
that they'll be able to use them in any and all configurations and it's
part of the DRM contract to have userspace gracefully handle validation
failures when they occur.

Valdiation occurs as part of DC and this in particular affects RV, so
disable this in dcn10_global_validation.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>