drm/i915/mso: add splitter state check

For starters, we expect the state to be zero, as we don't enable MSO
anywhere.

v2: Refer to splitter.

Cc: Nischal Varide <nischal.varide@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/459a332f3cdce941c57312150872559db68f88c1.1614682842.git.jani.nikula@intel.com