drm/i915: Remove extraneous mm_switch in ppgtt enable

Originally this commit message said:
Now that do_switch does the mm switch, and we always enable the aliasing
PPGTT, and contexts at the same time, there is no need to continue doing
this during PPGTT enabling.

Since originally writing the patch however, I introduced the concept of
synchronous mm switching (using MMIO). Since this is generally not
recommended in the spec (for reasons unknown), I've isolated its usage
as much as possible. As such the "extraneous" switch only ever will
occur when we have full PPGTT.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>