drm/amdgpu: remove unsafe context releasing

If ctx was released between put and get, then "get" would crash.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>