drm/i915: Set crtc_state->active to false when CRTC is disabled (v2)

With the recent modeset internal rework, we wind up setting
crtc_state->enable to false, but leave crtc_state->active as true, which
is incorrect.  This mismatch gets caught by drm_atomic_crtc_check() and
causes subsequent atomic operations (such as plane updates while the
CRTC is disabled) to fail.

Bisect points to

        commit dad9a7d6d96630182fb52aae7c3856e9e7285e13
        Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
        Date:   Tue Apr 21 17:13:19 2015 +0300

            drm/i915: Use atomic helpers for computing changed flags

as the commit that actually triggers the regression.

v2: Update to alter in-flight state rather than already-committed state
    (first version was accidentally based on a midpoint of Ander's
    modeset rework series, before his final patches that add proper
    state swapping to the legacy modeset path).

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Testcase: igt/kms_universal_plane
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -12340,6 +12340,7 @@ static int __intel_set_mode(struct drm_crtc *modeset_crtc,
 			continue;
 
 		if (!crtc_state->enable) {
+			crtc_state->active = false;
 			intel_crtc_disable(crtc);
 		} else if (crtc->state->enable) {
 			intel_crtc_disable_planes(crtc);