drm/i915: Drop struct_mutex from suspend state save/restore

struct_mutex provides no serialisation of the registers and data
structures being saved and restored across suspend/resume. It is
completely superfluous here.

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/20191004134015.13204-19-chris@chris-wilson.co.uk