drm/radeon: use RCU query for GEM_BUSY syscall

We don't need to call the (expensive) radeon_bo_wait, checking the
fences via RCU is much faster. The reservation done by radeon_bo_wait
does not save us from any race conditions.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>