drm/i915: change coding style of the write_infoframe functions

Don't use intermediate variables, change the value of 'val' as we go
through the function. The new style looks more similar to the rest of
our code. IMHO, it's also easier to read and change.

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