drm/radeon: Only call drm_vblank_on/off between drm_vblank_init/cleanup

Prevents the

	if (WARN_ON(pipe >= dev->num_crtcs))

in drm_vblank_on/off from triggering if acceleration fails to
initialize, in which case we call drm_vblank_cleanup.

Reported-and-Tested-by: Julian Margetson <runaway@candw.ms>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>