drm/amd/gfx11: move the gfx mutex into the caller

Otherwise we can fail to drop the software mutex when
we fail to take the hardware mutex.

Fixes: 76acba7b ("drm/amdgpu/gfx11: add a mutex for the gfx semaphore")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>