tests/kms_hdmi_inject: Provide igt_descriptions

This test is quite simple which makes it perfect for the first real
world example of igt_describe usage.

v2: keep the original comment (Simon)

Cc: Simon Ser <simon.ser@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>