drm/i915: Implement HDCP for HDMI

This patch adds HDCP support for HDMI connectors by implementing
the intel_hdcp_shim.

Nothing too special, just a bunch of DDC reads/writes.

Changes in v2:
- Rebased on drm-intel-next
Changes in v3:
- Initialize new worker
Changes in v4:
- Remove SKL_ prefix from most register names (Daniel)
- Wrap sanity checks in WARN_ON (Daniel)
- Consolidate the enable/disable functions into one toggle fn
- Use intel_hdcp_init (Daniel)
Changes in v5:
- checkpatch whitespace nits
Changes in v6:
- None

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180108195545.218615-9-seanpaul@chromium.org