drm/client: Fix memory leak in drm_client_modeset_probe

In the Linux kernel, the following vulnerability has been resolved:

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