drm/i915: Don't override output type for DDI HDMI

Currently a DDI port may register the DP hotplug handler even though
it's used with HDMI, and the DP HPD handler overrides the encoder
type forcibly to DP.  This caused the inconsistency on a machine
connected with a HDMI monitor; upon a hotplug event, the DDI port is
suddenly switched to be handled as a DP although the same monitor is
kept connected, and this leads to the erroneous blank output.

This patch papers over the bug by excluding the previous HDMI encoder
type from this override.  This should be fixed more fundamentally,
e.g. by moving the encoder type reset from the HPD or by having
individual encoder objects for HDMI and DP.  But since the bug has
been present for a long time (3.17), it's better to have a
quick-n-dirty fix for now, and keep working on a cleaner fix.

Bugzilla: http://bugzilla.opensuse.org/show_bug.cgi?id=955190
Fixes: 0e32b39c

 ('drm/i915: add DP 1.2 MST support (v0.7)')
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1447931396-19147-1-git-send-email-tiwai@suse.de
Signed-off-by: Jani Nikula <jani.nikula@intel.com>