drm/i915: Fix DPT suspend/resume on !HAS_DISPLAY platforms

The drm.mode_config state is not initialized in case of !HAS_DISPLAY
so taking the fb_lock and iterating the fb list won't work on those
platforms. Skip the suspend/resume with an explicit check for this.

Fixes: 9755f055

 ("drm/i915: Restore memory mapping for DPT FBs across system suspend/resume")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211125171603.1775179-1-imre.deak@intel.com