drm/i915/dp: Simplify the DSC slice config loop's slices-per-pipe iteration

drm/i915/dp: Simplify the DSC slice config loop's slices-per-pipe iteration

Simplify the slice config loop in intel_dp_dsc_get_slice_count(), using
the loop iterator as the slices-per-pipe value directly, instead of
looking up the same value from an array.

While at it move the code comment about the slice configuration closer
to where the configuration is determined and clarify it a bit.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260114162232.92731-11-imre.deak@intel.com