drm/i915: kill lpt pch transcoder->crtc mapping code for fifo underruns

It's racy: There's no guarantee that we won't walk this code (due to a
pch fifo underrun interrupt) while someone is changing the pointers
around.

The only reason we do this is to use the righ crtc for the pch fifo
underrun accounting. But we never expose this to userspace, so
essentially no one really cares if we use the "wrong" crtc.

So let's just rip it out.

With this patch fifo underrun code will always use crtc A for tracking
underruns on the (only) pch transcoder on LPT.

v2: Add a big comment explaining what's going on. Requested by Paulo.

v3: Fixup spelling in comment as spotted by Paulo.

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