drm/i915/crtc: replace I915_STATE_WARN_ON() with I915_STATE_WARN()

Describe the assertion better.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512110444.1448231-2-jani.nikula@intel.com