drm/i915/dsi: abstract afe_clk calculation

We'll make more use of it in the future.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0341fdc13260625150315b0b57a4227eb766c50f.1575974743.git.jani.nikula@intel.com