drm/fb-helper: fix sphinx markup for DRM_FB_HELPER_DEFAULT_OPS

Fix invalid sphinx markup in the comment for the newly added
DRM_FB_HELPER_DEFAULT_OPS.

Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1475692454-11543-1-git-send-email-contact@stefanchrist.eu