drm: Refactor vblank sequence number comparison

Move the repeated (a - b) <= (1 << 23) to its own function.

v2: Catch the '1<<23' inside drm_handle_vblank() as well

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322100650.26082-1-chris@chris-wilson.co.uk


Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>