drm/i915/selftests: Properly reset mock object propers for each test

I forgot to do this properly in

commit 6f11f374
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jul 23 10:34:55 2021 +0200

    drm/plane: remove drm_helper_get_plane_damage_clips

intel-gfx CI didn't spot this because we run each selftest in each own
invocations, which means reloading i915.ko. But if you just run all
the selftests in one go at boot-up, then it falls apart and eventually
we cross over the hardcoded limited of how many properties can be
attached to a single object.

Fix this by resetting the property count. Nothing else to clean up
since it's all static storage anyway.
Reported-and-tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Fixes: 6f11f374 ("drm/plane: remove drm_helper_get_plane_damage_clips")
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021202048.2638668-1-daniel.vetter@ffwll.ch

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

@@ -30,6 +30,7 @@ static void mock_setup(struct drm_plane_state *state)
 	mock_device.driver = &mock_driver;
 	mock_device.mode_config.prop_fb_damage_clips = &mock_prop;
 	mock_plane.dev = &mock_device;
+	mock_obj_props.count = 0;
 	mock_plane.base.properties = &mock_obj_props;
 	mock_prop.base.id = 1; /* 0 is an invalid id */
 	mock_prop.dev = &mock_device;