drm: Add some HDCP related #defines

In preparation for implementing HDCP in i915, add some HDCP related
register offsets and defines. The dpcd register offsets will go in
drm_dp_helper.h whereas the ddc offsets along with generic HDCP stuff
will get stuffed in drm_hdcp.h, which is new.

Changes in v2:
- drm_hdcp.h gets MIT license (Daniel)
Changes in v3:
- None
Changes in v4:
- None
Changes in v5:
- None
Changes in v6:
- SPDX license

Cc: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ramalingam C <ramalingm.c@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180108195545.218615-5-seanpaul@chromium.org