drm/scdc: declare drm_scdc_get_scrambling_status

Fix sparse warning:

drivers/gpu/drm/drm_scdc_helper.c:138:6: warning: symbol
'drm_scdc_get_scrambling_status' was not declared. Should it be static?

Fixes: 62c58af3 ("drm/edid: detect SCDC support in HF-VSDB")
Cc: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490193218-24806-1-git-send-email-jani.nikula@intel.com