drm/amdgpu/display: Unlock mutex on error

commit ee434a4f upstream.

Make sure we pass through ret label to unlock the mutex.

Signed-off-by: John van der Kamp <sjonny@suffe.me.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>