drm/doc: Improve kernel-doc for drm_fb_cma_helper.c

drm/doc: Improve kernel-doc for drm_fb_cma_helper.c

kernel-doc was unhappy, appease it.

v2: Resolve typo Liviu spotted through more consistency (aka
copy-pasting from drm_helper_mode_fill_fb_struct).

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: laurent.pinchart@ideasonboard.com
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464729075-22243-3-git-send-email-daniel.vetter@ffwll.ch