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

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: 79e6770cb1f5 ("drm/i915: Remove obsolete ringbuffer emission for gen8+")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Matthew Auld <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]