drm/i915: Perform pageflip using mmio if the GPU is terminally wedged

After a hang and failed reset, we cannot use the GPU to execute the page
flip instructions. Instead we can force a synchronous mmio flip. (Later,
we can reduce the synchronicity of the mmio flip by moving some of the
delays off to a worker, like the current page flip code; see vblank
tasks.)

References: https://bugs.freedesktop.org/show_bug.cgi?id=72631


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>