drm/client: Fix memory leak in drm_client_modeset_probe

When a new mode is set to modeset->mode, the previous mode should be freed.
This fixes the following kmemleak report:

drm_mode_duplicate+0x45/0x220 [drm]
drm_client_modeset_probe+0x944/0xf50 [drm]
__drm_fb_helper_initial_config_and_unlock+0xb4/0x2c0 [drm_kms_helper]
drm_fbdev_client_hotplug+0x2bc/0x4d0 [drm_kms_helper]
drm_client_register+0x169/0x240 [drm]
ast_pci_probe+0x142/0x190 [ast]
local_pci_probe+0xdc/0x180
work_for_cpu_fn+0x4e/0xa0
process_one_work+0x8b7/0x1540
worker_thread+0x70a/0xed0
kthread+0x29f/0x340
ret_from_fork+0x1f/0x30

cc: <stable@vger.kernel.org>
Reported-by: Zhang Yi <yizhan@redhat.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230711092203.68157-3-jfalempe@redhat.com