drm/i915/display: Apply interactive priority to explicit flip fences

Currently, if a modeset/pageflip needs to wait for render completion to
an object, we boost the priority of that rendering above all other work.
We can apply the same interactive priority boosting to explicit fences
that we can unwrap into a native i915_request (i.e. sync_file).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119204454.10343-1-chris@chris-wilson.co.uk