drm/i915: Don't rely that 2 VDSC engines are always enough for pixel rate

We are currently having FIFO underruns happening for kms_dsc test case,
problem is that, we check if curreny cdclk is >= pixel rate only if
there is a single VDSC engine enabled(i.e dsc_split=false) however if
we happen to have 2 VDSC engines enabled, we just kinda rely that this
would be automatically enough.
However pixel rate can be even >= than VDSC clock(cdclk) * 2, so in that
case even with 2 VDSC engines enabled, we still need to tweak it up.
So lets compare pixel rate with cdclk * VDSC engine count and
check if it still requires bumping up.
Previously we had to bump up CDCLK many times for similar reasons.

v2: - Use new intel_dsc_get_num_vdsc_instances to determine number of VDSC
      engines, instead of slice count(Ankit Nautiyal)
v3: - s/u8/int/ (Jani Nikula)
v4: - Remove slice count mentions(Ankit Nautiyal)
    - Use DIV_ROUND_UP in order to make sure that resulting CDCLK would
      be always >= than required, after division(Ankit Nautiyal)

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/20230704131758.14024-3-stanislav.lisovskiy@intel.com