drm/i915: Call non-locking version of drm_kms_helper_poll_enable(), v2

drm_kms_helper_poll_enable() is called from a context in
intel_hpd_irq_storm_disable() where the the mode_config mutex is
already locked.
When this function was converted to lock this mutex in
commit 8c4ccc4a ("drm/probe-helper: Grab mode_config.mutex
in poll_init/enable") a deadlock occurred.
Call the newly implemented non-locking version of this function.

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>