drm/i915: Pull some watermarks state into a separate structure

There is a bunch of global state that needs to be considered when
checking watermarks for validity. Move most of that to a new
structure intel_wm_config, to avoid having to pass around so
many variables.

One notable thing left out is the DDB partitioning information,
since we often anyway need to check the same watermarks against
both 1/2 and 5/6 DDB partitioning layouts.

v2: s/pipes_active/num_pipes_active

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>