Revert "drm/i915: Remove unneeded hack now for CDCLK"

Revert "drm/i915: Remove unneeded hack now for CDCLK"

This reverts commit be7bd6025e660967962c70ef450f0065f88585a5.

Unfortunately according to our recent findings there is still some
unidentified factor, requiring CDCLK to be set higher - otherwise we
still get underruns on some multipipe configurations, despite CDCLK
being set according to BSpec formula. So getting again back into debug
mode to indentify the cause, meanwhile setting CDCLK=Pixel rate back in
order to remove regression in 10% of the cases due to FIFO underruns.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: 7ee15926458f ("drm/i915: Adjust CDCLK accordingly to our DBuf bw needs")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200608065552.21728-1-stanislav.lisovskiy@intel.com