drm/i915: Update intel_crtc_active() to use state values (v2)

With the switch to atomic plumbing for planes, some of our commit-time
work (e.g., watermarks) is done after the new atomic state is swapped
into the relevant DRM object, but before the DRM core has a chance to
update its legacy state values.  Switch intel_crtc_active() to look at
the state objects rather than legacy fields to ensure we operate on the
proper values.

Note that we're continuing to use intel_crtc->active here for the time
being since crtc->state isn't really hooked up yet.  Once CRTC states
are wired up properly, we'll want to switch this over to use
crtc->state->active instead.

v2: Switch back to intel_crtc->active for now; when Ander's work on CRTC
    states is ready, we can flip this over to use crtc->state->active
    instead. (Ville)

Cc: Ander Conselvan De Oliveira <conselvan2@gmail.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>