drm/i915/dp: Limit max compressed bpp to 18 when forcing DSC

drm/i915/dp: Limit max compressed bpp to 18 when forcing DSC

While forcing DSC when it is not actually required, the max compressed
bpp from the source gets picked for compression for some pipe bpps.
Apparently, when DSC is not required, forcing DSC to the maximum compressed
bpp seems to cause underruns.

To avoid this, limit the max compressed bpp to 18 while forcing DSC. This
value works across platforms with different pipe bpps and prevents
underruns.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Naga Venkata Srikanth V <nagavenkata.srikanth.v@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030084100.1361689-1-ankit.k.nautiyal@intel.com