drm/amd/display: Use DRM provided page flip flag.

Starting with 4.12 kernel DRM provides page flip flags so we
don't need to have our own copy.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>