drm/tests: hdmi: Replace '[_]MHz' with 'mhz'

drm/tests: hdmi: Replace '[_]MHz' with 'mhz'

Improve consistency throughout drm_hdmi_state_helper_test.c by replacing
the two occurrences of '[_]MHz' substring with 'mhz'.

As a bonus, this also helps getting rid of checkpatch.pl complaint:

  CHECK: Avoid CamelCase: <reject_100_MHz_connector_hdmi_funcs>

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250527-hdmi-conn-yuv-v5-7-74c9c4a8ac0c@collabora.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>