drivers: drm: cadence: select DRM_DISPLAY_HDMI_HELPER if Cadence HDCP driver is enabled

According to commit 4fc8cb47

("drm/display: Move HDMI
Helpers into display-helper module "), it needs to select
DRM_DISPLAY_HDMI_HELPER if Cadence HDCP driver is enabled.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>