drm/i915/gtt: Remove obsolete switch_mm hooks for gen8+

As the ppgtt for execlists is tightly coupled to the executing context,
and not switch separately, we no longer use the ppgtt->switch_mm hooks
on gen8+. Remove them.

References: 79e6770c

 ("drm/i915: Remove obsolete ringbuffer emission for gen8+")
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/20180604131552.29370-1-chris@chris-wilson.co.uk