drm/i915/dsc: Add helper for writing DSC Selective Update ET parameters

drm/i915/dsc: Add helper for writing DSC Selective Update ET parameters

There are slice row per frame and pic height configuration in DSC Selective
Update Parameter Set 1 register. Add helper for configuring these.

v2:
  - Add WARN_ON_ONCE if vdsc instances per pipe > 2
  - instead of checking vdsc instances per pipe being > 1 check == 2

Bspec: 71709
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patch.msgid.link/20260304113011.626542-4-jouni.hogander@intel.com