drm/i915/dp: Fetch downstream port info if needed during DPCD fetch

v2: Fix parenthesis mismatch, spotted by Jani Nikula

Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Fixup merge conflict and MAX_DOWNSTREAM #define as spotted by
Jani.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>