drm/radeon: Remove redundant fence unref in pageflip path.

Not needed anymore, as it is already unreffed within
radeon_flip_work_func() after its only use.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>