drm/i915: Don't read dpcd for disconnected ports

drm/i915: Don't read dpcd for disconnected ports

Reading from disconnected ports will spit out timeout error
on the dmesg. Skip the attempted read if the port is not
connected and avoid confusing users/testcases about
expected timeouts.

This new dpcd debugfs entry was introduced by commit efddb9b2d10b
("drm/i915: add i915 specific connector debugfs file for DPCD")

v2 by Jani: move the check at the top, out of the loop.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90060
Tested-by: yex.tian@intel.com
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>