This makes drm_get_encoder_name() thread safe.

Reference: http://lkml.kernel.org/r/645ee6e22cad47d38a2b35c21c8d5fe3@DC1-MBX-01\
.ptsecurity.ru
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>