commit 9a11d2e7 upstream.

virtio_gpu was failing to send vblank events when using the atomic IOCTL
with the DRM_MODE_PAGE_FLIP_EVENT flag set. This patch fixes each and
enables atomic pageflips updates.
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[ kamal: backport to 4.2-stable: older crtc api ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>