drm/radeon/radeon_connectors.c: add a NULL test before dereference

The encoder variable can be NULL in this function so I believe it should
be checked before dereference.

Coverity CID: 13253

[airlied: extremely unlikely to happen]

Signed-off-by: Darren Jenkins <darrenrjenkins@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>