drm/i915/debugfs: HDCP capability enc NULL check

DP-MST connector encoder initializes at modeset
Adding a connector->encoder NULL check in order to
avoid any NULL pointer dereference.
intel_hdcp_enable() already handle this but debugfs
can also invoke the intel_{hdcp,hdcp2_capable}.
Handling it gracefully.

v2:
- Use necessary lock and NULL check in
  i915_hdcp_sink_capability_show. [Imre]

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210211140502.22786-1-anshuman.gupta@intel.com