drm/amd/display: Prevent freesync power optimization during validation

[Why]
Due to the freesync power optimization increasing vtotal, the driver
believes that MPO is supported. MPO is turned on. Freesync then
exits, MPO can no longer be supported and immediate flip fails. This
causes corruption on the panel.

[How]
Avoid the freesync optimization when doing validation so in the case
freesync exits, driver can still support the configuration.

Signed-off-by: Isabel Zhang <isabel.zhang@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>