drm/tests: modes: Fix drm_display_mode memory leak

drm/tests: modes: 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_modes_analog_tv_mono_576i() 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: e31538489908 ("drm/tests: Add tests for the new Monochrome value of tv_mode")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250408-drm-kunit-drm-display-mode-memleak-v1-6-996305a2e75a@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>