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