drm/amdgpu: Unpin BO if we can't get fences in amdgpu_crtc_page_flip

If reservation_object_get_fences_rcu failed, we'd previously go directly
to the cleanup label, so we'd leave the BO pinned.

While we're at it, remove two amdgpu_bo_unreserve calls in favour of two
new labels.

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>