drm: Advertise async page flip ability through GETCAP ioctl

Let applications know whether the kernel supports asynchronous page
flipping.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>

diff --cc drivers/gpu/drm/drm_crtc.c
Simple merge

diff --cc drivers/gpu/drm/drm_ioctl.c
Simple merge

diff --cc include/drm/drm_crtc.h
Simple merge

diff --cc include/uapi/drm/drm.h
Simple merge