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 aa7471d2
("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: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>