drm/i915: asserts for lvds pre_enable

Lots of bangin my head against the wall^UExperiments have shown that
we really need to enable the lvds port before we enable plls. Strangely
that seems to include the fdi rx pll on the pch.

Note that the pch pll assert can fire since the lvds port has it's own
special clock source settings in the DPLL register, which means it
will never have a shared dpll (since there's only one LVDS port).

Anyway, encode this new evidence with a few nice WARNs.

v2: Incorporate review comments from Imre.
- Explain why lvds can't have a shared dpll.
- Update the WARN output.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>