drm/doc: fix drm_driver_legacy_fb_format

Didn't get updated in a rework of the original patch.

Fixes: 059b5eb5 ("drm: move native byte order quirk to new drm_driver_legacy_fb_format function")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-20-daniel.vetter@ffwll.ch