drm/i915/vdsc: Introduce 3rd VDSC engine VDSC2

Introduce the register bits to enable the 3rd DSC engine VDSC2.
Add support to read/write these bits.

v2: Only introduce bits that are used and update the subject and commit
message. (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/20241030041036.1238006-5-ankit.k.nautiyal@intel.com