drm/i915: Kill intel_crtc->vbl_wait

Share the waitqueue that drm_irq uses when performing the vblank evade
trick for atomic pipe updates.

v2: Keep intel_pipe_handle_vblank() (Chris)

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