drmCommandWriteRead gives us a -errno, and we only checked for -1 (-EPERM,
incidentally). All the callers wanted 0 for errors, which they were
getting by the fact that req.value was 0-initialized in our stack
allocation (though this only works as long as the kernel doesn't return an
error after setting req.value to something), and -EPERM not really being
an answer we would expect from an ioctl at this stage in the driver.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2769>
487576e3cf