drm/tests: modeset: Fix drm_display_mode memory leak

drm/tests: modeset: Fix drm_display_mode memory leak

drm_mode_find_dmt() returns a drm_display_mode that needs to be
destroyed later one. The drm_test_pick_cmdline_res_1920_1080_60() 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: 8fc0380f6ba7 ("drm/client: Add some tests for drm_connector_pick_cmdline_mode()")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250408-drm-kunit-drm-display-mode-memleak-v1-2-996305a2e75a@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>