drm/i915: make DBLCLK modes work

They require an AVI InfoFrame with a proper Pixel Repetition field.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45729


Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>