drm/i915: Check for id==PLANE_CURSOR instead of type==DRM_PLANE_TYPE_CURSOR

The VLV/CHV watermark calculation is really interested in the hardware
plane type rather than the plane type (which is more of a software
concept). Let's check plane->id rather plane->type.

No functional changes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303151928.23053-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>