drm: Add a non-locking version of drm_kms_helper_poll_enable(), v2

drm_kms_helper_poll_enable() was converted to lock the mode_config
mutex in commit 8c4ccc4a


("drm/probe-helper: Grab mode_config.mutex in poll_init/enable").

This disregarded the cases where this function is called from a context
where this mutex is already locked.

Add a non-locking version as well.

Changes since v1:
- use function name suffix '_locked' for the function that
  is to be called from a locked context.

Signed-off-by: Egbert Eich <eich@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>