drm/i915/dsi: Don't do DSC horizontal timing adjustments in command mode

In the Linux kernel, the following vulnerability has been resolved: drm/i915/dsi: Don't do DSC horizontal timing adjustments in command mode Stop adjusting the horizontal timing values based on the compression ratio in command mode. Bspec seems to be telling us to do this only in video mode, and this is also how the Windows driver does things. This should also fix a div-by-zero on some machines because the adjusted htotal ends up being so small that we end up with line_time_us==0 when trying to determine the vtotal value in command mode. Note that this doesn't actually make the display on the Huawei Matebook E work, but at least the kernel no longer explodes when the driver loads. (cherry picked from commit 0b475e91ecc2313207196c6d7fd5c53e1a878525)