First of all don't try to read dpcd if PSR is not even supported.

But also, if read failed return -EIO instead of reporting via a
backchannel.

v2: fix dev_priv: At this level m->private is the connector. (CI/DK)
    don't convert dpcd read errors to EIO. (DK)

Fixes: 5b7b3086 ("drm/i915/psr: Split sink status into a separate debugfs node")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: NDhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180720003155.16290-1-rodrigo.vivi@intel.com
(cherry picked from commit 7a72c78bdd0a1ea1d879610542679cc680398220)
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>