drm/tests: Fix a test in drm_test_check_valid_clones()

The drm_atomic_get_crtc_state() function returns error pointers and not
NULL.  Update the check to check for error pointers as well as NULL.

Fixes: 88849f24 ("drm/tests: Add test for drm_atomic_helper_check_modeset()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/c50f11c7-932c-47dc-b40f-4ada8b9b6679@stanley.mountain


Signed-off-by: Maxime Ripard <mripard@kernel.org>