drm/connector: hdmi: Factor out bpc and format computation logic

drm/connector: hdmi: Factor out bpc and format computation logic

In preparation to support fallback to an alternative output format, e.g.
YUV420, when RGB cannot be used for any of the available color depths,
move the bpc try loop out of hdmi_compute_config() and, instead, make it
part of hdmi_compute_format(), while adding a new parameter to the
latter holding the output format to be checked and eventually set.

Since this helper now also changes hdmi.output_bpc in addition to
hdmi.output_format, highlight the extended functionality by renaming it
to hdmi_compute_format_bpc().

This improves code reusability and further extensibility, without
introducing any functional changes.

Reviewed-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-v5-5-74c9c4a8ac0c@collabora.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>