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):

cc: <stable@vger.kernel.org>
Fixes: 1d42bbc8 ("drm/fbdev: fix cloning on fbcon")
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/20230711092203.68157-2-jfalempe@redhat.com

drivers/gpu/drm/drm_client_modeset.c

@@ -311,6 +311,9 @@ static bool drm_client_target_cloned(struct drm_device *dev,
 	can_clone = true;
 	dmt_mode = drm_mode_find_dmt(dev, 1024, 768, 60, false);
 
+	if (!dmt_mode)
+		goto fail;
+
 	for (i = 0; i < connector_count; i++) {
 		if (!enabled[i])
 			continue;
@@ -326,11 +329,13 @@ static bool drm_client_target_cloned(struct drm_device *dev,
 		if (!modes[i])
 			can_clone = false;
 	}
+	kfree(dmt_mode);
 
 	if (can_clone) {
 		DRM_DEBUG_KMS("can clone using 1024x768\n");
 		return true;
 	}
+fail:
 	DRM_INFO("kms: can't enable cloning when we probably wanted to.\n");
 	return false;
 }