drm/xe/vf: Don't check CTC_MODE[0] if VF

Starting from commit 18778b5f ("drm/xe: Eliminate usage of
TIMESTAMP_OVERRIDE") we access the CTC_MODE register only to warn
if it has undocumented value.  There is no point in doing that on
the VF driver.  While here, move this check to a helper function.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311114042.1954-2-michal.wajdeczko@intel.com