drm: add drm_get_connector_force_name

Follow the naming in debugfs also for logging, add "unknown" for values
beyond the enumerated ones.

v2: add \n in connector_show, make internal to drm (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487580708-29340-1-git-send-email-jani.nikula@intel.com