drm/tests: add drm_hdmi_state_helper_test MODULE_DESCRIPTION()

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/tests/drm_hdmi_state_helper_test.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Fixes: eb66d34d ("drm/tests: Add output bpc tests")
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619-md-drm-tests-drm_hdmi_state_helper_test-v1-1-41c5fe2fdb4a@quicinc.com