drm/i915: Add rudimentary plane state verification

Check that the planes are in the state we expect them to be. For
now we can only check whether each plane is correctly enabled or
disabled. In the future we may want to expand the plane state
readout to support a more thorough verification.

v2: Verify all planes part of the state as long as at least
    one crtc is doing a modeset (Daniel)
v3: Fix typoes (James)

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-11-ville.syrjala@linux.intel.comSigned-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -11600,6 +11600,18 @@ verify_crtc_state(struct drm_crtc *crtc,
 	}
 }
 
+static void
+intel_verify_planes(struct intel_atomic_state *state)
+{
+	struct intel_plane *plane;
+	const struct intel_plane_state *plane_state;
+	int i;
+
+	for_each_new_intel_plane_in_state(state, plane,
+					  plane_state, i)
+		assert_plane(plane, plane_state->base.visible);
+}
+
 static void
 verify_single_dpll_state(struct drm_i915_private *dev_priv,
 			 struct intel_shared_dpll *pll,
@@ -12393,6 +12405,9 @@ static void intel_atomic_commit_tail(struct drm_atomic_state *state)
 		intel_modeset_verify_crtc(crtc, state, old_crtc_state, new_crtc_state);
 	}
 
+	if (intel_state->modeset)
+		intel_verify_planes(intel_state);
+
 	if (intel_state->modeset && intel_can_enable_sagv(state))
 		intel_enable_sagv(dev_priv);