drm/i915: hw state readout support for pixel_multiplier

Incomplete since ilk+ support needs proper pch dpll tracking first.
SDVO get_config parts based on a patch from Jesse Barnes, but fixed up
to actually work.

v2: Make sure that we call encoder->get_config _after_ we
get_pipe_config to be consistent in both setup_hw_state and the
modeset state checker. Otherwise the clever trick with handling the
pixel mutliplier on i915G/GM where the encoder overrides the default
value of 1 from the crtc get_pipe_config function doesn't work.
Spotted by Imre Deak.

v3: Actually cross-check the pixel mutliplier (but not on pch split
platforms for now). Now actually also tested on a i915G with a sdvo
encoder plugged in.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>