drm/i915: Enable dithering on NatSemi DVO2501 for Fujitsu S6010

This patch enables the (unfortunately undocumented) scaler of the
NatSemi 2501 DVO found in the Fujitsu-Siemens S6010 laptop and other
machines of the same series and age.

Parts of the DVO scaler logic have been revealed by reverse
engineering and trial and error, so your milage may vary. The
patch (and the whole ns2501 DVO code) is currently only good for
the 1024x768 panel of the S6010, and may hopefully work on other
machines with the same panel size.

The mode-specific configuration of the scaler have been moved out
into a separate class, the mode-agnostic settings remain as raw
register list as their purpose remains unclear at this point.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Thomas Richter <thor@math.tu-berlin.de>
[danvet: Make the thing apply and conform to kernel patch
expectations.]
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>