drm/i915/overlay: Drop struct_mutex guard

The overlay uses the modeset mutex to control itself and only required
the struct_mutex for requests, which is now obsolete.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004134015.13204-16-chris@chris-wilson.co.uk