drm/i915/dsi: remove unused dsi_rr_formula()

The dsi_rr_formula() function has been unused for almost two years,
since

commit 44d4c6ee


Author: Shobhit Kumar <shobhit.kumar@intel.com>
Date:   Tue Dec 10 12:14:56 2013 +0530

    drm/i915: Compute dsi_clk from pixel clock

citing the reason as pixel clock based calculation being recommended in
the MIPI host controller documentation. Remove the dead code, we can
always bring it back if it's needed.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452249940-2605-2-git-send-email-jani.nikula@intel.com