drm/i915: Drop WaDisablePSDDualDispatchEnable:ivb for IVB GT2

Both Bspec and the W/A database state that WaDisablePSDDualDispatchEnable
is only needed for IVB GT1.

The only real confusion here is that the the W/A database also says to
write to the GT2 only register as well, which is strange if the W/A is
only for GT1.

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