drm/atomic: Clean up wait_for_vblanks, v2.

Stop relying on a per crtc_state last_vblank_count, we shouldn't touch
crtc_state after commit. Move it to atomic_state->crtcs.

Also stop re-using new_crtc_state->enable, we can now simply set a
bitmask with crtc_crtc_mask.

Changes since v1:
- Keep last_vblank_count in __drm_crtc_state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/8e4759a4-24d3-3f80-bd1a-1e7a9c83b612@linux.intel.com