drm/i915: Amalgamate gen6_mm_switch() and vgpu_mm_switch()

These are identical, so let's just use the same vfunc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467618513-4966-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>