drm/i915/hdcp: No HDCP when encoder is't initialized

There can be situation when DP MST connector is created without
mst modeset being done, in those cases connector->encoder will be
NULL. MST connector->encoder initializes after modeset.
Don't enable HDCP in such cases to prevent any crash.

Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Juston Li <juston.li@intel.com>
Tested-by: Karthik B S <karthik.b.s@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111081120.28417-5-anshuman.gupta@intel.com