drm/doc: Include new drm_blend.c

There's not much point in kerneldoc if it's not included:
- It won't show up in the pretty html pages.
- The comments itself won't get parsed, which means 0day won't pick up
  changes, resulting in stale docs fast.

Also, uapi really should be core, not helpers, so move drm_blend.c to
that. That also means that the zpos normilize function loses it's
helper status (and we might as well call it always). For that,
EXPORT_SYMBOL. Just spotted while integrating docs and noticing that
one was missing.

With sphinx there's really no excuse any more to not build the docs
and make sure it's all nice!

$ make DOCBOOKS="" htmldocs

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-15-git-send-email-daniel.vetter@ffwll.ch