drm/doc: Task to rename CMA helpers

drm/doc: Task to rename CMA helpers

I'm kinda fed up explaining why the have a confusing name :-)

v2: Fix typo that Eric Engestrom spotted.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129132153.28844-1-daniel.vetter@ffwll.ch