[Intel-gfx] i915 fixes

[Chris Wilson]

At the moment, only the first patch ("drm/i915: Initialise g4x watermarks for
disabled pipe") looks ready. Keith, I think this reflects the consensus
we reached for that particular patch; I have the second stage of
unifying the identical ironlake/g4x routines pending for -next.

The second two patches do fix a critical issue we have with generation
of PGTBL_ER upon initialisation and Daniel reported it improving the
stability of resume on his 855GM. However, the initialisation code is
quite hairy and these need lots of review and widespread testing.

Thanks,
-Chris