xf86drm: don't fatal on per device error in drmGetDevice[s]2

When iterating over all the device nodes if drmProcessPciDevice()
returned an error for any node the function would return an error,
ignoring any valid nodes.

The result of this on OpenBSD where drmProcessPciDevice() results in
device nodes being opened to issue ioctls to get pci data
was that data obtained from /dev/drm0 would be ignored if /dev/drm1
could not be opened.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>