drm/i915: Only recalculate wm's for planes part of the state, v2.

Only planes that are part of the state should be used for recalculating
watermarks. For planes not part of the state the previous patch allows
us to re-use the old values since they're calculated even for levels
that are not actively used.

Changes since v1:
- Remove big if from intel_crtc_atomic_check.
- Remove extra newline.
- Remove memset in ilk_compute_pipe_wm.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456826842-32553-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>