drm/i915: Don't re-compute pipe watermarks except for the affected pipe

No point in re-computing the watermarks for all pipes, when only one
pipe has changed. The watermarks stored under intel_crtc.wm.active are
still valid for the other pipes. We just need to redo the merging.

We can also skip the merge/update procedure completely if the new
watermarks for the affected pipe come out unchanged.

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