drm/i915: Extract mm switching to function

In order to do the full context switch with address space, it's
convenient to have a way to switch the address space. We already have
this in our code - just pull it out to be called by the context switch
code later.

v2: Rebased on BDW support. Required adding BDW.

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