drm/i915: split the dpll clock compute out from display vtable.

this single function might be possible to merge later, but
for now it's simple to just split it out.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ba570aa10b694b2e8640e0c58430fd0053c306b7.1632869550.git.jani.nikula@intel.com