drm/i915: move intel_update_lvds to intel_lvds->pre_pll_enable

A few things needed to change:
- HAS_PCH_SPLIT since ilk+ is not yet converted to this.
- s/LVDS/intel_lvds->reg/ to prep for ilk conversion
- replace the clock.p2 == 7 check with a is_dual_link check
- s/adjusted_mode/intel_lvds->fixed_mode

v2: Rebase on top of Jani Nikula's panel rework. I'm wondering whether
we shouldn't add an attached_panel pointer to intel_encoder, to
replace the encoder private ->attached_connector pointers, since
that's essentially what we need.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>