drm/i915: set default value for config->pixel_multiplier

This way we can simplify the code quite a bit.

Also add a WARN in the sdvo code to complain about a bogus value
and kill the readout code in intel_ddi.c that Jesse sneaked in.
HW state readout for the pixel multiplier will work a bit differently
in the end.

v2: Rebase on top of the fdi pixel mutliplier handling fix.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>