drm: fix page_flip error handling

Free event and restore event_space only when page_flip->flags has
DRM_MODE_PAGE_FLIP_EVENT if page_flip() is failed.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>