drm/i915: extract hw ppgtt setup/cleanup code

At the moment only cosmetics, but being able to initialize/cleanup
arbitrary ppgtt address spaces paves the way to have more than one of
them ... Just in case we ever get around to implementing real
per-process address spaces. Note that in that case another vfunc for
ppgtt would be beneficial though. But that can wait until the code
grows a second place which initializes ppgtts.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>