drm/i915/dsi: Fix state mismatch warns for horizontal timings with DSC

drm/i915/dsi: Fix state mismatch warns for horizontal timings with DSC

When DSC is enabled consider the compression ratio that was used during
horizontal timing calculations.

This may still lead to warns due to rounding errors in the round-trip.

v2 by Jani:
- rebase on top of the more generic dsc state readout

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c2481aaf67ea396aa4698cd2d8e23d19ec4f4ecf.1575974743.git.jani.nikula@intel.com