drm/radeon: Complete page flip even if waiting on the BO fence fails

Otherwise the DRM core and userspace will be confused about which BO the
CRTC is scanning out.

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>