drm/i915: Precompute static ddi_pll_sel values in encoders

This way only the dynamic WRPLL selection for hdmi ddi mode is
done in intel_ddi_pll_select.

v2: Don't clobber the precomputed values when selecting clocks fro
hdmi encoders.
v3 (from Paulo): Rebase on top of the s/IS_HASWELL/HAS_DDI/ patch.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>