drm/i915/dg2: add TRANS_DP2_CTL register definition

This register controls the DP 2.0 datapath.

Bspec: 69967
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0d3529df4501c5dd8ad1da0b6dbaabcfa97510b4.1629735412.git.jani.nikula@intel.com