drm/i915/gtt: Remove vgpu check for gen6

Since vgpu is not supported on Haswell or any other gen6/7, we do not
need to check and act upon it's enablement.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608150435.15010-2-chris@chris-wilson.co.uk