drm/kms: add page flipping ioctl

This adds a page flipping ioctl to the KMS API.  The ioctl takes an fb ID
and a ctrc ID and flips the crtc to the given fb at the next vblank.
The ioctl returns immediately but the flip doesn't happen until after
any rendering that's currently queued up against the new framebuffer
is done.  After submitting a page flip, any execbuffer involving the
old front buffer will block until the flip is completed.

Optionally, a vblank event can be generated when the swap eventually
happens.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>