drm/i915: Convert i915_gem_object_attach_phys() to ww locking, v2.

Simple adding of i915_gem_object_lock, we may start to pass ww to
get_pages() in the future, but that won't be the case here;
We override shmem's get_pages() handling by calling
i915_gem_object_get_pages_phys(), no ww is needed.

Changes since v1:
- Call shmem put pages directly, the callback would
  go down the phys free path.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-10-maarten.lankhorst@linux.intel.com