drm/i915: Prefer CRTC 'active' rather than 'enabled' during WM computations

Only the intel_crtc->active is accurate at the point where we wish to
perform WM computations, so use it instead of crtc->enabled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>