drm/i915: De-magic the PSR AUX message

Use pack_aux() to construct the PSR exit DPMS D0 AUX message,
and use the defines from dp_dp_helper.h to populate the message
contents.

v2: Use sizeof() for message size (Jani)
    Use a generic loop to write EDP_PSR_AUX_DATA registers

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>