drm/radeon: return -ENOENT in fence_wait_next v2

We should signal the caller that we haven't waited at all.

v2: only change fence_wait_next not fence_wait_last.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>