drm/client: Fix memory leak in drm_client_target_cloned

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

drm/client: Fix memory leak in drm_client_target_cloned

dmt_mode is allocated and never freed in this function.

It was found with the ast driver, but most drivers using generic fbdev

setup are probably affected.

This fixes the following kmemleak report:

backtrace:

[<00000000b391296d>] drm_mode_duplicate+0x45/0x220 [drm]

[<00000000e45bb5b3>] drm_client_target_cloned.constprop.0+0x27b/0x480 [drm]

[<00000000ed2d3a37>] drm_client_modeset_probe+0x6bd/0xf50 [drm]

[<0000000010e5cc9d>] __drm_fb_helper_initial_config_and_unlock+0xb4/0x2c0 [drm_kms_helper]

[<00000000909f82ca>] drm_fbdev_client_hotplug+0x2bc/0x4d0 [drm_kms_helper]

[<00000000063a69aa>] drm_client_register+0x169/0x240 [drm]

[<00000000a8c61525>] ast_pci_probe+0x142/0x190 [ast]

[<00000000987f19bb>] local_pci_probe+0xdc/0x180

[<000000004fca231b>] work_for_cpu_fn+0x4e/0xa0

[<0000000000b85301>] process_one_work+0x8b7/0x1540

[<000000003375b17c>] worker_thread+0x70a/0xed0

[<00000000b0d43cd9>] kthread+0x29f/0x340

[<000000008d770833>] ret_from_fork+0x1f/0x30

unreferenced object 0xff11000333089a00 (size 128):