drm/amd/display: Add check for vrr_active_fixed

Why:
vrr_active_fixed should also be checked when
determining if DRR is in use

How:
Add check for vrr_active_fixed when allow_freesync
and vrr_active_variable are also checked

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Austin Zheng <austin.zheng@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>