drm/i915: Accept alloc_size == blocks

If the minimum required ddb space for all the planes equals the
total ddb space available we are allowed to use the relevant
watermark level.

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-2-ville.syrjala@linux.intel.com


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