drm/i915: shovel compute clock into crtc->config.dpll on ilk

This was somehow lost in the pipe_config->dpll introduction in

commit f47709a9


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Mar 28 10:42:02 2013 +0100

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

While at it, extract a few small helpers for common computations.

v2: Use the newly added helpers more thanks to Ville's trick to
typedef the legacy intel_clock_t as the new-world struct dpll.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>