drm/i915: Drop struct_mutex guard from debugfs/framebuffer_info

It protects nothing being accessed for the intel_framebuffer, so it's
own locking had better be sufficient.

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/20191004134015.13204-17-chris@chris-wilson.co.uk