Add CRTC ID to vblank event

When using the atomic API, one request can span multiple CRTCs, however
one event is generated per CRTC. As we cannot disambiguate the CRTC with
user data (since we only have one piece of user data to pass in), newer
kernels can include the CRTC ID in the page flip event.

Add a new vfunc to dispatch vblank events carrying a CRTC ID to clients
who negotiate a higher interface version.

[daniels: Rebased, include new cap, call page_flip_handler if it is set
          but page_flip_handler2 isn't even on newer contexts, write a
	  commit message.]

v2: Split into separate commit.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>