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: be7f5fc ("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 da9b1c6)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: aknautiyal

drivers/gpu/drm/i915/display/intel_dp.c

@@ -1050,10 +1050,11 @@ u8 intel_dp_dsc_get_slice_count(const struct intel_connector *connector,
 		u8 test_slice_count = valid_dsc_slicecount[i] * num_joined_pipes;
 
 		/*
-		 * 3 DSC Slices per pipe need 3 DSC engines,
-		 * which is supported only with Ultrajoiner.
+		 * 3 DSC Slices per pipe need 3 DSC engines, which is supported only
+		 * with Ultrajoiner only for some platforms.
 		 */
-		if (valid_dsc_slicecount[i] == 3 && num_joined_pipes != 4)
+		if (valid_dsc_slicecount[i] == 3 &&
+		    (!HAS_DSC_3ENGINES(display) || num_joined_pipes != 4))
 			continue;
 
 		if (test_slice_count >