drm/i915: silence useless messages about DDI buffer translation

These messages are not really useful since it's very easy to check
which mode is used for each port: The values programmed are based on
the port type, then assigned to the ddi_translations variable.
Currently we use DP mode for ports A-D and FDI mode for port E.

Also, when we add the code to enable/disable PC8+,
intel_prepare_ddi_buffers will be called more often and will eat your
dmesg buffers.

While at it, fix the coding style of the "for" statement above.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Pimp commit message with Paulo's more detailed explanation of
how the ddi translation buffer settings are computed, to answer a
question from Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>