drm/i915/gtt: Remove redundant hsw_mm_switch()

hsw_mm_switch() and gen7_mm_switch() are identical, so let's remove the
redundant specialism.

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/20180608150435.15010-1-chris@chris-wilson.co.uk