drm/i915/dp: Check for HAS_DSC_3ENGINES while configuring DSC slices

DSC 12 slices configuration is used for some specific cases with
Ultrajoiner. This can be supported only when each of the 4 joined pipes
have 3 DSC engines each.

Add the missing check for 3 DSC engines support before using 3 DSC
slices per pipe.

Fixes: be7f5fcd ("drm/i915/dp: Enable 3 DSC engines for 12 slices")
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Cc: <stable@vger.kernel.org> # v6.14+
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250414024256.2782702-3-ankit.k.nautiyal@intel.com


(cherry picked from commit da9b1c61)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>