radeon: correct RADEON_GEM_WAIT_IDLE use

RADEON_GEM_WAIT_IDLE is declared DRM_IOW but libdrm
uses it with drmCommandWriteRead instead of drmCommandWrite
which leads to the ioctl being unmatched and returning an
error on at least OpenBSD.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>