drm/i915: Do not rely on wm preservation for ILK watermarks

The original intent was to preserve watermarks as much as possible
in intel_pipe_wm.raw_wm, and put the validated ones in intel_pipe_wm.wm.

It seems this approach is insufficient and we don't always preserve
the raw watermarks, so just use the atomic iterator we're already using
to get a const pointer to all bound planes on the crtc.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102373


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: stable@vger.kernel.org #v4.8+
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171019151341.4579-1-maarten.lankhorst@linux.intel.com