drm: expose subpixel order name routine v3

Just like we have for connector type etc.

v2: drop static array (Chris)
v3: add kdoc (Daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>