intel_hdmi_bpc_possible() is used by the DP code as well where
the native HDMI source limits do not apply. So let's split this
into a pair of functions: one for the source vs. one for the sink.

This is basically reverting some of commit 41828125 ("drm/i915:
Move platform checks into intel_hdmi_bpc_possible()") slightly,
but in a nicer form. I guess I forgot at the time that the DP side
uses this too.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015133921.4609-2-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>