drm/i915/dp: read DPCD PSR capability only on eDP

Reduce AUX transactions for non-eDP.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>