We have an implementation requirement that precludes the user from
requesting a ggtt entry when the device is operating in ppgtt mode. Move
the current check from inside the execbuffer object collation to the
prevalidation phase.

v2: Roll both invalid flags checks into one
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>