Allows using atomic flip helpers for drivers
using ASYNC flip.
Remove ASYNC_FLIP restriction in helpers and
caches the page flip flags in drm_crtc_state
to be used in the low level drivers.

v2:
Resending the patch since the original was broken.

v3:
Save flag in crtc_state instead of plane_state

v4:
Reset the flag before using again.

v5:
Fix type in header.
Rename the field to pageflip_flags.
Remove unrelated hunk.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486072591-3893-2-git-send-email-Andrey.Grodzovsky@amd.com