drm/i915: Update crtc state active flag based on DPMS

In a follow up patch the function that computes mode changes will be
replaced with the one from the atomic helpers. To preserve the behavior
of legacy modeset forcing DPMS on, that function will need to detect a
change in the active state of the crtc, so that has to be kept up to
date.

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