drm/i915: Fix > vs >= mismatch in watermark/ddb calculations

Bspec says we have to reject the watermark if it's >= the ddb
allocation. Fix the code to reject the == case as it should.
For transition watermarks we can just use >=, for the rest
we'll do +1 when calculating the minimum ddb allocation size.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181221171436.8218-5-ville.syrjala@linux.intel.com


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