drm/udl: Implement page_flip ioctl

This is a very crude page_flip implementation for UDL. There are ways
to make it better (make it asynchronous, make it do actual vsynced
flips...) but that's for another patch.

[airlied: fixup primary change]

Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>