drm/i915: Fix bogus dig_port_map[] assignment for pre-HSW

The recent commit [0bdf5a05: drm/i915: Add reverse mapping between
port and intel_encoder] introduced a reverse mapping to retrieve
intel_dig_port object from the port number.  The code assumed that the
port vs intel_dig_port are 1:1 mapping.  But in reality, this was a
too naive assumption.

As Martin reported about the missing HDMI audio on his SNB machine,
pre-HSW chips may have multiple intel_dig_port objects corresponding
to the same port.  Since we assign the mapping statically at the init
time and the multiple objects override the map, it may not match with
the actually enabled output.

This patch tries to address the regression above.  The reverse mapping
is provided basically only for the audio callbacks, so now we set /
clear the mapping dynamically at enabling and disabling HDMI/DP audio,
so that we can always track the latest and correct object
corresponding to the given port.

Fixes: 0bdf5a05

 ('drm/i915: Add reverse mapping between port and intel_encoder')
Reported-and-tested-by: Martin Kepplinger <martink@posteo.de>
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456324522-21591-1-git-send-email-tiwai@suse.de
(cherry picked from commit 9dfbffcf

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>