drm/i915: prepare HDMI link for Haswell

On Haswell, we need to properly train the DDI buffers prior to enabling
HDMI, and enable the required clocks with correct dividers for the desired
frequency.

Also, we cannot simple reuse HDMI routines from previous generations of
GPU, as most of HDMI-specific stuff is being done via the DDI port
programming instead of HDMI-specific registers.

This commit take advantage of the WR PLL clock table which is in a
separate (previous) commit to select the right divisors for each mode.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>