drm/i915/display: Move chunks of code out of bxt_set_cdclk()

No functional change. Moving segments out to simplify
bxt_set_cdlck()

v2: s/bxt_cdclk_pll/bxt_cdclk_pll_update (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221025223042.138810-3-anusha.srivatsa@intel.com