drm/i915: Enable HDMI on ValleyView

HDMI register offsets are different in Valleyview. Add support for the
same.

v2: drop superfluous comments in HDMI init (Daniel)

Signed-off-by: Beeresh G <beeresh.g@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Reviewed-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>