drm/i915/dsi: refactor bitrate calculations in intel_dsi_vbt_init()

Abstract bitrate calculation to a newly resurrected intel_dsi.c file
that will contain common code for VLV and ICL DSI.

No functional changes.

Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/100e9721dfdec4f3987549ef24291bafc9cb0517.1539613303.git.jani.nikula@intel.com