drm/amdgpu: release correct lock in amdgpu_gfx_enable_kgq()

This function was releasing the incorrect lock on the error path.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 1156e1a6 ("drm/amdgpu: add [en/dis]able_kgq() functions")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>