drm/i915: Move platform checks into intel_hdmi_bpc_possible()

Let's put the platform checks into intel_hdmi_bpc_possible() so that
it'll confirm both the source and sink capabilities.

Cc: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210511160532.21446-4-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>