[intel-gem] Move domains to relocation records. add set_domain ioctl.

[intel-gem] Move domains to relocation records. add set_domain ioctl.

Domain information is about buffer relationships, not buffer contents. That
means a relocation contains the domain information as it knows how the
source buffer references the target buffer.

This also adds the set_domain ioctl so that user space can move buffers to
the cpu domain.