drm/i915: Split the BDW device definition to prepare for dual BSD rings on BDW GT3

Based on the hardware spec, the BDW GT3 has the different configuration
with the BDW GT1/GT2. So split the BDW device info definition.
This is to do the preparation for adding the Dual BSD rings on BDW GT3 machine.

V1->V2: Follow Daniel's comment to pay attention to the stolen check for BDW
in kernel/early-quirks.c

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>