drm/atomic: Rename crtc_state->pageflip_flags to async_flip

It's the only flag anyone actually cares about. Plus if we're unlucky,
the atomic ioctl might need a different flag for async flips. So
better to abstract this away from the uapi a bit.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: David Francis <David.Francis@amd.com>
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903190642.32588-3-daniel.vetter@ffwll.ch