drm/i915: create pipe_config->dpll for clock state

Clock computations and handling are highly encoder specific, both in
the optimal clock selection and also in which clocks to use and when
sharing of clocks is possible.

So the best place to do this is somewhere in the encoders, with a
generic fallback for those encoders without special needs. To facility
this, add a pipe_config->clocks_set boolean.

This patch here is only prep work, it simply sets the computed clock
values in pipe_config->dpll, and uses that data in the hw clock
setting functions.

Haswell code isn't touched, simply because Haswell clocks work much
different and need their own infrastructure (with probably a
Haswell-specific config->ddi_clock substruct).

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>