drm/i915: properly alternate between DVI and HDMI

drm/i915: properly alternate between DVI and HDMI

This solves problems that happen when you alternate between HDMI and
DVI on the same port. I can reproduce these problems using DP->HDMI
and DP->DVI adapters on a DP port.

When you first plug HDMI and then plug DVI, you need to stop sending
DIPs, even if the port is in DVI mode (see the HDMI register spec). If
you don't stop sending DIPs, you'll see a pink vertical line on the
left side of the screen, some modes will give you a black screen, some
modes won't work correctly.

When you first plug DVI and then plug HDMI, you need to properly
enable the DIPs, otherwise the HW won't send them. After spending a
lot of time investigating this, I concluded that if the DIPs are
disabled, we should not write to the DIP register again because when
we do this, we also set the AVI InfoFrame frequency to "once", and
this seems to really confuse our hardware. Since this problem was not
exactly easy to debug, I'm adopting the defensive behavior and not
just avoing the "disable twice" sequence, but also explicitly
selecting the AVI InfoFrame and setting its frequency to a correct
one.

Also, move the "is_dvi" check from intel_set_infoframe to the
set_infoframes functions since now they're going to be the first ones
to deal with the DIP registers.

This patch adds the code to fix the problem, but it depends on the
removal of some code that can't be removed right now and will come
later in the patch series. The patch that we need is:
  - drm/i915: don't write 0 to DIP control at HDMI init

[danvet: Paulo clarified that this additional patch is only required
to make the fix complete, this patch here alone doesn't introduce a
regression but only partially solves the problem of randomly clearing
the dip registers.]

V2: Be even more defensive by selecting AVI and setting its frequency
outside the "is_dvi" check.

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