drm/i915/dp: Fix DSC sink's slice count capability check

drm/i915/dp: Fix DSC sink's slice count capability check

A DSC sink supporting DSC slice count N, not necessarily supports slice
counts less than N. Hence the driver should check the sink's support for
a particular slice count before using that slice count, fix
intel_dp_dsc_get_slice_count() accordingly.

Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251215192357.172201-4-imre.deak@intel.com