drm/i915: Replace bxt_clk_div with struct dpll

bxt_clk_div is basically the same as struct dpll. Just use the latter.

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/20220307233940.4161-6-ville.syrjala@linux.intel.com