drm/i915: explicitly cast pipe -> cpu_transcoder

This makes sparse happy and also makes it a bit more obvious where we
pull off this trick - after all we're only allowed to do it eithe as a
default or on platforms where there is no disdinction between the pipe
and the cpu transcoder.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>