drm/tests: probe-helper: Fix drm_display_mode memory leak

drm/tests: probe-helper: Fix drm_display_mode memory leak

drm_analog_tv_mode() and its variants return a drm_display_mode that
needs to be destroyed later one. The
drm_test_connector_helper_tv_get_modes_check() test never does however,
which leads to a memory leak.

Let's make sure it's freed.

Reported-by: Philipp Stanner <phasta@mailbox.org>
Closes: https://lore.kernel.org/dri-devel/a7655158a6367ac46194d57f4b7433ef0772a73e.camel@mailbox.org/
Fixes: 1e4a91db109f ("drm/probe-helper: Provide a TV get_modes helper")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250408-drm-kunit-drm-display-mode-memleak-v1-7-996305a2e75a@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>