drm/i915/skl: New ddb allocation algorithm

This patch implements new DDB allocation algorithm as per HW team
recommendation. This algo takecare of scenario where we allocate less DDB
for the planes with lower relative pixel rate, but they require more DDB
to work.
It also takes care of enabling same watermark level for each
plane in crtc, for efficient power saving.

Changes since v1:
 - Rebase on top of Paulo's patch series

Changes since v2:
 - Fix the for loop condition to enable WM

Changes since v3:
 - Fix crash in cursor i-g-t reported by Maarten
 - Rebase after addressing Paulo's comments
 - Few other ULT fixes
Changes since v4:
 - Rebase on drm-tip
 - Added separate function to enable WM levels
Changes since v5:
 - Fix a crash identified in skl-6770HQ system
Changes since v6:
 - Address review comments from Matt
Changes since v7:
 - Fix failure return in skl_compute_plane_wm (Matt)
 - fix typo
Changes since v8:
 - Always check cursor wm enable irrespective of total_data_rate
Changes since v9:
 - fix typo

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170601055918.4601-1-mahesh1.kumar@intel.com