Incorrect drmIoctl() return value and code check in drmmode_show_cursor()

When drmModeSetCursor2() call was replaced with bare drmIoctl() call in 92df7097, a bug was introduced. Previously, return value from drmIoctl() was mangled: negative return values were replaced with -errno by the following wrapper function as defined in the xf86drmMode.c:

static inline int DRM_IOCTL(int fd, unsigned long cmd, void *arg)
{ 
    int ret = drmIoctl(fd, cmd, arg);
    return ret < 0 ? -errno : ret;
}

Now, this mangling no longer happens, and the check if (ret == -EINVAL) after the drmIoctl() call is wrong. It should be replaced with if (ret == -1 && errno == EINVAL), which is both correct and follows POSIX semantics for ioctl(2)-type functions.

The bug can manifest itself as HW cursor becoming invisible after update from version 18.1.0. See this FreeBSD bug report for details and discussion.