drm/radeon: Move pinning the BO back to radeon_crtc_page_flip()

As well as enabling the vblank interrupt. These shouldn't take any
significant amount of time, but at least pinning the BO has actually been
seen to fail in practice before, in which case we need to let userspace
know about it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>