drm/i915: add basic pipe config dump support

All this pipe config abstraction adds another layer of complexity, so
it's good to have better visibility into what's going on exactly.
Doesn't dump out everything yet, and some bits are a bit duplicated
but this should be a good start.

Note that at boot-up a lot of the fields are 0 even for enabled pipes,
this is simply because our hw state readout code doesn't support
everything.

v2: Remove a few more now redudant debug output lines.

v3: Review from Paulo
- use transcoder_name
- fix up format specifiers
- add missing ':' in debug output

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