drm/i915: Check hdcp key loadability

HDCP1.4 key can be loaded, only when Power well #1 is enabled and cdclk
is enabled. Using the I915 power well infrastruture, above requirement
is verified.

This patch enables the hdcp initialization for HSW, BDW, and BXT.

v2:
  Choose the PW# based on the platform.
v3:
  No Changes.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1522663834-24482-4-git-send-email-ramalingam.c@intel.com