drm/i915/dp: Enable 3 DSC engines for 12 slices

Certain resolutions require 12 DSC slices support along with ultrajoiner.
For such cases, the third DSC Engine per Pipe is enabled. Each DSC
Engine processes 1 Slice, resulting in a total of 12 VDSC slices
(4 Pipes * 3 DSC Instances per Pipe).
Add support for 12 DSC slices and 3 DSC engines for such modes.

v2: Add missing check for 3 slices support only with 4 joined pipes.
(Suraj)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030041036.1238006-8-ankit.k.nautiyal@intel.com