drm/i915: Add helper function for getting number of VDSC engines

Currently we are using dsc_split and bigjoiner variables for determining
amount of VDSC instances, however that might change in future, if we happen
to have more of those.
So lets pack all that logic into single function for convenience, so that
at least this isn't hardcoded throughout the whole VDSC code.

v2: - s/u8/int/ (Jani Nikula)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230704131758.14024-2-stanislav.lisovskiy@intel.com