drm: Conslidate blending properties in drm_blend.[hc]

Imo zpos, rotatation, blending eq (once we have it) and all that
should be in drm_blend.c, since those are all about how exactly the
pixels are rendered onto the CRTC's visible area. Also noticed that
one exported function accidentally ended up in drm_crtc_internal.h,
move it to the right place too.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474448370-32227-3-git-send-email-daniel.vetter@ffwll.ch