drm/i915: calculate watermarks for devices that have 3 pipes

This adds proper support for calculating those watermarks, checking for
number of available pipes instead of specific GPU variants when deciding
if watermarks for 3rd pipe are necessary.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>