drm/i915/hdcp: Add encoder check in hdcp2_get_capability

Add encoder check in intel_hdcp2_get_capability to avoid
null pointer error.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722064451.3610512-3-suraj.kandpal@intel.com