drm/i915/vdsc: Add support for read/write PPS for 3rd DSC engine

With BMG each pipe has 3 DSC engines, so add bits to read/write the PPS
registers for the 3rd DSC engine

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/20241030041036.1238006-6-ankit.k.nautiyal@intel.com