Revert "drm/i915/skl: New ddb allocation algorithm"

This reverts commit bb9d85f6.

New ddb allocation algorithm is a show stopper on my SKL system.

Besides not be able to get external DP 4k@60 (through USB type C),
It fully hang my screen when unplugging the USB type C.

Bugzilla: https://patchwork.freedesktop.org/patch/161571/
Fixes: bb9d85f6 ("drm/i915/skl: New ddb allocation algorithm")
Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497376350-3400-1-git-send-email-rodrigo.vivi@intel.com