drm/i915: Remove unneeded hack now for CDCLK

No need to bump up CDCLK now, as it is now correctly
calculated, accounting for DBuf BW as BSpec says.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519131117.17190-8-stanislav.lisovskiy@intel.com