drm/i915/dsi: account for DSC in horizontal timings

When DSC is enabled, we need to adjust the horizontal timings to account
for the compressed (and therefore reduced) link speed.

The compressed frequency ratio simplifies down to the ratio between
compressed and non-compressed bpp.

Bspec: 49263
Suggested-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
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/fecebdc2719dd0c78eaf8f4d3225bb185956d7db.1575974743.git.jani.nikula@intel.com