drm: Document drm_cache interface

Notice that this uncovers an issue with the kernel-doc handling of array
arguments, causing the first parameter of drm_clflush_pages() to not
show up in the rst-generated page.  A proposed fix is under review in
linux-doc: <http://www.spinics.net/lists/linux-doc/msg42544.html

>

Changes since v1:
 - Add section to drm-mm.rst.
 - Fix kernel-doc style issues.
 - s/memory/kernel memory/.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109215649.6860-1-krisman@collabora.co.uk