drm/i915/dp: Add missing slice count check during mode validation

drm/i915/dp: Add missing slice count check during mode validation

Add the missing check for a valid slice count during
mode validation when DSC is enabled.

Cc: Vinod Govindapillai <vinod.govindapillai@intel.com>
Fixes: 745395b51c26 ("drm/i915/dp: Add intel_dp_mode_valid_with_dsc()")
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260216070421.714884-2-imre.deak@intel.com
(cherry picked from commit ec4db429fd38e5c5cbea3521049739fd2718845c)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>