drm/i915: Make sure cursor has enough ddb for the selected wm level

We use a fixed ddb allocation for the cursor. Now the calculation
actually makes sure we have enough ddb space, but let's double check
anyway.

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-6-ville.syrjala@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>