mock a drm_plane in igt_check_plane_state to make the test more robust

igt_check_plane_state test crashes in drm_atomic_helper_check_plane_state
when trying to de-reference drm_plane_state->plane->dev
due to the lack of a struct drm_plane in the mock struct drm_plane_state.
Since drm_plane_state always should contain a plane, the mock also
needs a plane to be the test more robust and realistic. Add it.
Signed-off-by: Claudio Suarez <cssk@net-c.es>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YaiWvQESctDMipjh@gineta.localdomain

drivers/gpu/drm/selftests/test-drm_plane_helper.c

@@ -87,11 +87,15 @@ int igt_check_plane_state(void *ignored)
 				DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC)
 		},
 	};
+	struct drm_plane plane = {
+		.dev = NULL
+	};
 	struct drm_framebuffer fb = {
 		.width = 2048,
 		.height = 2048
 	};
 	struct drm_plane_state plane_state = {
+		.plane = &plane,
 		.crtc = ZERO_SIZE_PTR,
 		.fb = &fb,
 		.rotation = DRM_MODE_ROTATE_0