drm/i915: add HDMI and DP port enumeration on ValleyView

ValleyView is similar to IbexPeak here, but with different register
offsets.

v2: use SDVOB instead ov VLV_HDMIB (Daniel)
    drop unnecessary eDP check in DP_C init (Daniel)

eDP support will be coming later from Shobit.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>