drm/i915: s/TRANSCONF/PCH_TRANSCONF/

Every time I read hsw code I get completely confused about this. So
call it what it is more explicitly.

Also, add an LPT_TRANSCONF for the pch transcoder A and use it in
lpt-only code, to really unconfuse me.

v2: s/plane/pipe/ in the TRANSCONF #define (Paulo).

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