drm/prime: fix potential race in drm_gem_map_detach

Unpin the GEM object only after freeing the sg table.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227115000.4105-1-christian.koenig@amd.com