For PRIME bo, we need to use uncached render targets so that any writes
are flushed out to main memory where they can be immediately read by a
PCI device. For simplicity, we just request that PRIME bo be also
SCANOUTs as that ensures that they will be created with the right
attributes for coherent main memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>