Reported by Coverity.  The intention is that the return value is
checked, but let's be more paranoid and make it extremely obvious
if something forgets to.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

Note to self: more sleep
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

This is a wrapper around the interfaces defined in an earlier commit,
and is also used by various userspace (either by a libdrm backend, or
libpciaccess) tools/tests.

In the future this will be extended to handle channels, replacing some
long-unloved code we currently use, and allow fifo/display/mpeg (hi
Ilia ;)) engines to all be exposed in the same way.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>