drm/tests: helpers: Create kunit helper to destroy a drm_display_mode

drm/tests: helpers: Create kunit helper to destroy a drm_display_mode

A number of test suites call functions that expect the returned
drm_display_mode to be destroyed eventually.

However, none of the tests called drm_mode_destroy, which results in a
memory leak.

Since drm_mode_destroy takes two pointers as argument, we can't use a
kunit wrapper. Let's just create a helper every test suite can use.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250408-drm-kunit-drm-display-mode-memleak-v1-1-996305a2e75a@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>