drm/i915: Move has_hdmi_sink check into intel_hdmi_bpc_possible()

drm/i915: Move has_hdmi_sink check into intel_hdmi_bpc_possible()

We wish intel_hdmi_bpc_possible() to consider whether the sink
supports HDMI or just DVI when checking whether it'll support
HDMI deep color or not. This also takes care of the "force DVI"
property.

Cc: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210511160532.21446-3-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>