drm/i915: Add reverse mapping between port and intel_encoder

This patch adds a reverse mapping from a digital port number to
intel_encoder object containing the corresponding intel_digital_port.
It simplifies the query of the encoder a lot.

Note that, even if it's a valid digital port, the dig_port_map[] might
point still to NULL -- usually it implies a DP MST port.  Due to this
fact, the NULL check in each place has no WARN_ON() and just skips the
port.  Once when the situation changes in future, we might introduce
WARN_ON() for a more strict check.

Signed-off-by: Takashi Iwai <tiwai@suse.de>