drm/gem|prime|mm: Use recommened kerneldoc for struct member refs

I just learned that &struct_name.member_name works and looks pretty
even. It doesn't (yet) link to the member directly though, which would
be really good for big structures or vfunc tables (where the
per-member kerneldoc tends to be long).

Also some minor drive-by polish where it makes sense, I read a lot
of docs ...

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-5-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_mm.c

@@ -552,8 +552,8 @@ EXPORT_SYMBOL(drm_mm_replace_node);
  * objects to the roster, probably by walking an LRU list, but this can be
  * freely implemented. Eviction candiates are added using
  * drm_mm_scan_add_block() until a suitable hole is found or there are no
- * further evictable objects. Eviction roster metadata is tracked in struct
- * &drm_mm_scan.
+ * further evictable objects. Eviction roster metadata is tracked in &struct
+ * drm_mm_scan.
  *
  * The driver must walk through all objects again in exactly the reverse
  * order to restore the allocator state. Note that while the allocator is used