drm/amd/display: Optimize subvp and drr validation

Two issues existed:
1) Configs that support DRR, but have it disabled will fail subvp+vblank
validation incorrectly. Use subvp+vblank path for this case.
2) Configs that support DRR and have it enabled can use higher voltage level
than required if they also support subvp+vblank. Use lowest supported voltage
level for this case.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>