drm/dp: Fix read pointer for drm_dp_downsteam_debug()

Pass in the array and not a pointer to the array to drm_dp_dpcd_read().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720174532.23377-1-chris@chris-wilson.co.uk