drm/i915/dsi: Extract {vlv,bxt}_get_pclk()

Extract the state->freq computation for VLV/BXT DSI PLL
into small helpers. We want to use these during the state
computation as well.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-5-ville.syrjala@linux.intel.com