drm: Make blocking vblank wait return when the vblank interrupts get disabled

If there's a blocking vblank wait in progress while the vblank interrupt
gets disabled, the current code will just let the vblank wait time out.
Instead make it return immediately when vblank interrupts get disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>