drm/i915: Use transcoder as index to MIPI regs

Conceptually, the MIPI registers are addressed by the MIPI transcoder
index, not the pipe. It doesn't matter right now, because there's a
1:1 relationship between pipes and MIPI transcoders, but that change
allows us to break that link in the future

V1: Created new patch to address Damien's review comment.
Replacing _PIPE calls to _TRANSCODER calls
V2: Re-basing on patch 2
V3: Re-basing on patch 2
V4: Re-basing on patch 2
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>