drm/i915: Compute clocks earlier

Do the DPLL computation before fastset checks. This should
allow us to get rid of all that horrible fuzzy clock handling
for fastsets. Who knows how many bugs there are caused by our
state not actually matching what the hardware will generate.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-9-ville.syrjala@linux.intel.com