drm/i915: Add _TRANS2()

A new macro that is going to be added in a further patch will need to
adjust the offset returned by _MMIO_TRANS2(), so here adding
_TRANS2() and moving most of the implementation of _MMIO_TRANS2() to
it and while at it taking the opportunity to rename pipe to trans.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiya@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiya@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730224753.14907-2-jose.souza@intel.com