drm/nouveau/kms/nv50: Switch to using atomic helper for flip.

v2:
Update code after flip_flags moved from plane_state
to crtc_state

v5:
Rename to pageflip_flags.

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/1486072591-3893-3-git-send-email-Andrey.Grodzovsky@amd.com