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

drivers/gpu/drm/Makefile

@@ -13,7 +13,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
 		drm_trace_points.o drm_global.o drm_prime.o \
 		drm_rect.o drm_vma_manager.o drm_flip_work.o \
 		drm_modeset_lock.o drm_atomic.o drm_bridge.o \
-		drm_framebuffer.o drm_connector.o
+		drm_framebuffer.o drm_connector.o drm_blend.o
 
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_GEM_CMA_HELPER) += drm_gem_cma_helper.o
@@ -25,7 +25,7 @@ drm-$(CONFIG_AGP) += drm_agpsupport.o
 drm_kms_helper-y := drm_crtc_helper.o drm_dp_helper.o drm_probe_helper.o \
 		drm_plane_helper.o drm_dp_mst_topology.o drm_atomic_helper.o \
 		drm_kms_helper_common.o drm_dp_dual_mode_helper.o \
-		drm_simple_kms_helper.o drm_blend.o drm_modeset_helper.o
+		drm_simple_kms_helper.o drm_modeset_helper.o
 
 drm_kms_helper-$(CONFIG_DRM_LOAD_EDID_FIRMWARE) += drm_edid_load.o
 drm_kms_helper-$(CONFIG_DRM_FBDEV_EMULATION) += drm_fb_helper.o