Just loop over the possible bpc values instead of
using an ugly if construct.

A slight change in behaviour is that we now call
intel_hdmi_{source,sink}_bpc_possible() even for 8bpc,
but that is fine since 8bpc is always supported.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015133921.4609-8-ville.syrjala@linux.intel.comReviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>