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>