drm/i915/dp: Only apply the workaround if the select is still active

As we may try to power down the link at various times, it is not
necessarily still coupled with an encoder and so we must be careful not
to depend upon an operation that is only valid when the link is still
attached to a pipe.

Fixes regression in 5bddd17f.

Reported-and-tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org [after applying 5bddd17f]