drm/i915: drop a few really redundant WARNs in hsw mode_set

- Correct cpu->pch display matching is already check when we detect
  the PCH type at driver load.
- Plane/pipe state is already checked both when a) enabling, b)
  disabling and in c) the modeset state checker. No need to go
  overboard and also check it in in between a) and b).

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