drm/i915/dp: Refactor dsc_slice_count handling in intel_dp_mode_valid()

drm/i915/dp: Refactor dsc_slice_count handling in intel_dp_mode_valid()

Make dsc_slice_count closer to the block where it is used and promote it
from u8 to int. This aligns it with upcoming DSC bubble pixel-rate
adjustments, where the slice count participates in wider arithmetic.

Currently, for non-eDP (DP/DP_MST) cases  the slice count is computed only
inside intel_dp_dsc_mode_valid() and is not used by the caller. Once DSC
bubble handling is added, dp_mode_valid() will need access to its own local
slice count for non-eDP cases as well.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260202103731.357416-14-ankit.k.nautiyal@intel.com