drm/i915/dsc: Expose dsc sink max slice count via debugfs

Number of DSC slices can be shown in the DSC debugfs so that
test can take a call whether the configuration can support forcing
bigjoiner/ultrajoiner.

v2: used intel_dp_is_edp() as the parameter to
    drm_dp_dsc_sink_max_slice_count() (Jani N)

Reviewed-by: Nemesa Garg <nemesa.garg@intel.com> (v1)
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/3387


Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241213093008.2149452-1-swati2.sharma@intel.com