drm/i915: Add some more tracked state to intel_pipe_wm

intel_pipe_wm will be used to track the state in different stages
of the watermark update process. For that we need to keep a bit
more state in intel_pipe_wm.

We also need to separate the multi-pipe intel_wm_config computation
from ilk_compute_wm_parameters() as that one deals with the future
state, and we need the intel_wm_config to match the current hardware
state at the time we do the watermark merging for multiple pipes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Frob conflict.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>