drm/i915/display/dg2: Sanitize CD clock

In case of CD clock squashing the divider is always 1. We don't
need to calculate the divider in use so let's skip that for DG2.

v2: Drop unnecessary local variable (Ville)
v3: Avoid if-else structure (Ville)
[v4: vsyrjala: Fix cd2x divider calculation (Uma),
               Introduce has_cdclk_squasher()]

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211119131348.725220-3-mika.kahola@intel.com