drm/i915: Fix WARN_ON condition for cursor plane ddb allocation

In some cases like latency[level]==0, wm[level].res_lines>31,
min_ddb_alloc can be U16_MAX, exclude it from the WARN_ON.

v2: Specify the cases in which we hit U16_MAX, indentation (Ville)

Fixes: 10a7e07b ("drm/i915: Make sure cursor has enough ddb for the selected wm level")
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216080619.10945-1-vandita.kulkarni@intel.com