drm/nv17-nv4x: Attempt to init some external TMDS transmitters.

sil164 and friends are the most common, usually they just need to be
poked once because a fixed configuration is enough for any modes and
clocks, so they worked without this patch if the BIOS had done a good
job on POST. Display couldn't survive a suspend/resume cycle though.
Unfortunately, BIOS scripts are useless here.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>