drm: Don't try and disable an encoder that was never enabled

Prevents code that assumes that the encoder is active when asked to be
disabled from dying a horrible death.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>