drm/i915: Remove struct_mutex guard for debugfs/opregion

Having a struct_mutex around the read of a BIOS blob serves no purpose.

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/20191004134015.13204-18-chris@chris-wilson.co.uk