drm/i915: Reject DRM_MODE_FLAG_DBLCLK with DVI sinks

The code assumes that DRM_MODE_FLAG_DBLCLK means that we enable the
pixel repeat feature. That only works with HDMI since it requires
AVI infoframe to signal the information to the sink. Hence even if
the mode dotclock would be valid we cannot currently assume that
we can just ignore the DBLCLK flag. Reject it for DVI sinks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200108181242.13650-6-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>