drm/radeon: enforce use of radeon_get_ib_value when reading user cmd

When ever parsing cmd buffer supplied by userspace we need to use
radeon_get_ib_value rather than directly accessing the ib as the user
cmd might not yet be copied into the ib thus the parser might read
value that does not correspond to what user is sending and possibly
allowing user to send malicious command undected.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>