Move WaDisableDgMirrorFixInHalfSliceChicken5 to gen9_init_workarounds

v2: Added stepping check

v3: Removed unused register bitmap
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
[danvet: Bikesheds.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>