drm/i915: Start using plane scale factor for relative data rate

BSpec clearly instructs us to use plane scale factor when calculating
relative data rate to be used when allocating DDB blocks for each plane.
For some reason we use scale factor for data_rate calculation, which is
used for BW calculations, however we are not using it for DDB calculations.
So lets fix it as described in BSpec 68907.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Garg, Nemesa <nemesa.garg@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719104833.25366-1-stanislav.lisovskiy@intel.com