drm/i915/skl: Prevent unclaimed register writes on skylake.

I'm getting unclaimed register writes when checking the WM registers
after the crtc is disabled. So I would imagine those are guarded by
the crtc power well. Fix this by not reading out wm state when the
power well is off.

Cc: stable@vger.kernel.org # v4.3
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92181


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>