freedreno: make cmdstream bo's read-only to GPU

freedreno: make cmdstream bo's read-only to GPU

If nothing else, this will make problems with cmdstream getting blit
over with pixels easier to track down (ie. faults when it first happens
rather than strange failures later from corrupted cmdstream when a
stateobj is later reused).

(NOTE this somewhat depends on the kernel supporting the flag, and the
iommu implementation.  But the worst case is just that the cmdstream
ends up writeable as before.)

Signed-off-by: Rob Clark <robdclark@gmail.com>