drm/i915/bdw: Remove BDW preproduction W/As until C stepping.

Let's clean this a bit

v2: Rebase after other Mika's patch that removed some BDW production workarounds.
v3: Removed stepping info.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>