drm/i915: Split plane watermark parameters into a separate struct

Give a name to the plane watermark related data we have currently
stored under intel_plane->wm.

We also observe that this data is more or less the same that we have
in the hsw_pipe_wm_parameters structure, so use it there as well.

v2: Make pahole happier

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