drm/i915/dsc: Switch to using intel_dsc_line_slice_count()

drm/i915/dsc: Switch to using intel_dsc_line_slice_count()

By now all the places are updated to track the DSC slice configuration
in intel_crtc_state::dsc.slice_config, so calculate the slices-per-line
value using that config, instead of using
intel_crtc_state::dsc.slice_count caching the same value and remove
the cached slice_count.

v2: Rebase on latest drm-tip, converting another user of dsc.slice_count
    in intel_vdsc_min_cdclk().

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com> # v1
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260114162232.92731-7-imre.deak@intel.com