drm/i915: don't enumerate HDMID if an eDP panel is already active on the port

This prevents the HDMI detect functions from poking at an eDP
connected panel, which can lead to trouble.

[danvet: Note that we have some other reports of DP vs. HDMI fighting,
but the general case is a much bigger fish to fry.]

References: https://bugs.freedesktop.org/show_bug.cgi?id=42278
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>