drm/i915: Use a high priority wq for nonblocking plane updates

system_unbound_wq can't keep up sometimes and we get dropped frames.
Switch to a high priority variant.

Reported-by: Heinrich Fink <heinrich.fink@daqri.com>
Tested-by: Heinrich Fink <heinrich.fink@daqri.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910121347.22958-1-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>