i915 get_eld ops may return an error when no encoder is connected, and
currently we regard the error as fatal and skip the whole ELD
handling.  This ended up with the missing ELD update at unplugging.

This patch fixes the issue by treating the error as the unplugged
state, instead of skipping the rest.
Reported-by: Libin Yang <libin.yang@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.5
Signed-off-by: Takashi Iwai <tiwai@suse.de>