drm/i915/gem: Distinguish each object type

Separate each object class into a separate lock type to avoid lockdep
cross-contamination between paths (i.e. userptr!).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022144501.26486-1-chris@chris-wilson.co.uk