drm/i915/dp: Use intel_dp_dsc_get_slice_config()

drm/i915/dp: Use intel_dp_dsc_get_slice_config()

Simplify things by computing the detailed slice configuration using
intel_dp_dsc_get_slice_config(), instead of open-coding the same.

While at it add a TODO comment to intel_dp_dsc_compute_config() to
explore if it's worth increasing the number of VDSC stream engines used,
in order to reduce the minimum CDCLK required.

v2: Add a TODO comment to intel_dp_dsc_compute_config() to explore if
    it's worth increasing the number of slices in order to use a lower
    CDCLK. (Jouni)

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260114162232.92731-16-imre.deak@intel.com