drm/i915: Check crtc->active in intel_crtc_disable_planes

This was lost in

commit ce22dba9
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Tue Apr 21 17:12:56 2015 +0300

    drm/i915: Move toggling planes out of crtc enable/disable.

and we still need that crtc->active check since the overall modeset
flow doesn't yet take dpms state into account properly. Fixes WARNING
backtraces on at least bdw/hsw due to the ips disabling code being
upset about being run on a switched-off pipe.

We don't need a corresponding change on the enable side since with the
old setCrtc semantics we always force-enable the pipe after a modeset.
And the dpms function intel_crtc_control already checks for ->active.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>