drm/i915: transform INTEL_OUTPUT_* into an enum

drm/i915: transform INTEL_OUTPUT_* into an enum

Because I got annoyed that I had to document what values "int
ddi_personality" is supposed to hold.

A good side-effect of this change is that now the compilers can do
some additional checks on our code, which may prevent some bugs in the
future. A bad side-effect of this change is that now the compilers do
some additional checks on our code and complain when a switch
statement doesn't check for all possible values, so we need to add
"default" cases to all those switches. Hopefully, this may help
preventing confusions against DRM_MODE_CONNECTOR_* and
DRM_MODE_ENCODER_*.

I guess that just by looking at the patch, some people will think this
change is not worth its benefits. In this case, I don't really mind
dropping the patch.

Also, there's probably still a few more places where we can
s/int/enum intel_output_type/, but we can change that later, when we
spot the places.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Resolve conflict due to reordered patches.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>