drm/i915: Consolidate HDMI force_dvi handling

Move the force_dvi check to a single function that can be called from
both mode validation and compute_config().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200108181242.13650-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>