drm/i915: Fix IVB GT2 WaDisableDopClockGating and WaDisablePSDDualDispatchEnable

IVB GT2 has two registers for these things, and both must be written.

To add a bit more confusion both Bspec and the W/A database state that
WaDisablePSDDualDispatchEnable is only needed for IVB GT1, but the W/A
database also says to write even the second GT2 only register. So I
don't really know what the right thing here is.

Note that Bspec disagrees with the w/a database here, but Ville
confirmed (by asking Chris) that on gt1 the 2nd reg doesn't exist.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add note as requested by Rodrigo.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>