drm/i915: don't rely on previous values when setting LPT TRANSCONF

Because we already set all the bits we can set.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: apply by hand due to dropped patch. Also, obey my OCD a bit
and do a s/_TRANSACONF/TRANSCONF(TRANSCODER_A)/, makes it more
consisten with other lpt pch code imnsho ...]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>