drm/amdgpu: use dma_resv_get_singleton in amdgpu_pasid_free_cb

Makes the code a bit more simpler.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-15-christian.koenig@amd.com