drm/i915/gen9: Drop re-allocation of DDB at atomic commit (v2)

Now that we're properly pre-allocating the DDB during the atomic check
phase and we trust that the allocation is appropriate, let's actually
use the allocation computed and not duplicate that work during the
commit phase.

v2:
 - Significant rebasing now that we can use cached data rates and
   minimum block allocations to avoid grabbing additional plane states.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-11-git-send-email-matthew.d.roper@intel.com