drm/i915/dp: Update Comment for Valid DSC Slices per Line

For some platforms, the maximum slices per DSC engine is 4, while for
others it is 2. Update the comment to reflect this and clarify that
the 'valid_dsc_slicecount' list represents the valid number of slices
per pipe.

Currently, we are working with 1, and 2 slices per DSC engine,
which works for all platforms. With this the number of slices per pipe
can be 1,2 or 4 with different slice & DSC engine configuration.

Add a #TODO for adding support for 4 slices per DSC engine where
supported.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030041036.1238006-2-ankit.k.nautiyal@intel.com