etnaviv: Only use renderonly_get_handle for GEM handles.

etnaviv: Only use renderonly_get_handle for GEM handles.

Note that for requests for Prime FDs or flink names, we return handles to
the etanviv BO, not the scanout BO.  This is at least better than previous
behavior of returning GEM handles for a request for an FD or flink name.

And add an assert that renderonly_get_handle is only used for getting the
GEM handle.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>