drm/i915: Enable WA batch buffers for Gen9

This patch only enables support for Gen9, the actual WA will be
initialized in subsequent patches.

The WARN that we use to warn user if WA batch support is not available
for a particular Gen is replaced with DRM_ERROR as warning here doesn't
really add much value.

v2: include all infrastructure bits in this patch so that subsequent
changes only correspond the WA added (Chris)

v3: use updated macro.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>