drm/i915: Rework ppgtt init to no require an aliasing ppgtt

drm/i915: Rework ppgtt init to no require an aliasing ppgtt

Currently we abuse the aliasing ppgtt to set up the ppgtt support in
general. Which is a bit backwards since with full ppgtt we don't ever
need the aliasing ppgtt.

So untangle this and separate the ppgtt init from the aliasing
ppgtt. While at it drag it out of the context enabling (which just
does a switch to the default context).

Note that we still have the differentiation between synchronous and
asynchronous ppgtt setup, but that will soon vanish. So also correctly
wire up the return value handling to be prepared for when ->switch_mm
drops the synchronous parameter and could start to fail.

Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>