drm/radeon: fix prime teardown order

We unmapped imported DMA-bufs when the GEM handle was dropped, not when the
hardware was done with the buffere.

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