Extract intel_hdmi_bpc_possible() from intel_hdmi_deep_color_possible()
so that we can reuse it for mode validation.

Cc: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210511160532.21446-2-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>