agp: remove uid comparison as security check

In the face of containers and user namespaces, a uid==0 check for
security is not safe.  Switch to a capability check.

I'm not sure I picked the right capability, but this being AGP
CAP_SYS_RAWIO seemed to make sense.

Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>