drm/i915: disable sdvo pixel multiplier cross-check for HAS_PCH_SPLIT

We don't (yet) have proper pixel multiplier readout support on pch
split platforms, so the cross check will naturally fail.

v2: Fix spelling in the comment, spotted by Ville.

v3: Since the ordering constraint is pretty tricky between the crtc
get_pipe_config callback and the encoder->get_config callback add a
few comments about it. Prompted by a discussion with Chris Wilson on
irc about why this does work anywhere else than on i915g/gm.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>