drm/i915: implement WaDisablePSDDualDispatchEnable on IVB & VLV

Workaround for dual port PS dispatch on GT1.

v2: pull in register definition & offset handling
v3: use IVB GT1 macro to get the right regs (Ben)
v4: add for VLV too (Ben)
v5: don't read the reg, it's masked so we'll only enable the one extra bit (Chris)
v6: use a _GT2 suffix for the second reg (Chris)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>