drm/i915: Eliminate {vlv,bxt}_configure_dsi_pll()

Fold the DSI PLL configuration functions into the DSI PLL
enable functions since they are small and not called from anywhere else.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460488478-18311-4-git-send-email-ville.syrjala@linux.intel.com


Tested-by: Jani Nikula <jani.nikula@intel.com>