drm/amd/display: Correcting prefetch mode for fast validate

[WHY and HOW]
When fast validating, we should be agnostic to any sort of optimizations
such as supporting Pstate or stutter in Vblank. We should change our
prefetch mode for fast validate to support none of these optimizations.
For example, Valve Index running at 144Hz can only be validated without
any support these optimizations.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Saaem Rizvi <SyedSaaem.Rizvi@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>