drm/i915: Detect panel's hdcp capability

DP HDCP1.4 spec mandates that An can be written to panel only after
detecting the panel's hdcp capability.

For DP 0th Bit of Bcaps register indicates the panel's hdcp capability
For HDMI valid BKSV indicates the panel's hdcp capability.

For HDMI it is optional to detect the panel's hdcp capability before
An Write.

v2:
  Added comments explaining the need for action [Seanpaul].
  Made panel's hdcp capability detection optional for hdmi [Seanpaul].
  Defined a func for reading bcaps for DP [Seanpaul].

v3:
  Removed the NULL initialization [Seanpaul].

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1517609350-10698-7-git-send-email-ramalingam.c@intel.com