drm/i915: Introduce gpu_reset_clobbers_display()

Factor out the "does the GPU reset clobber the display?" check into a
small helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470428910-12125-4-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>