drm/i915: Use named initializers for gmch wm params

Using names initializers when filling out the watermark structs
saves you from having go look up the struct definition every
single time.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>