drm/tests: hdmi: Rename max TMDS rate fallback tests

drm/tests: hdmi: Rename max TMDS rate fallback tests

In preparation to extend the max TMDS rate fallback tests for covering
YUV420 output, update the rather generic function names
drm_test_check_max_tmds_rate_{bpc|format}_fallback() to properly
indicate the intended test cases.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20250527-hdmi-conn-yuv-v5-16-74c9c4a8ac0c@collabora.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>