drm/i915: Init PPGTT before context enable

commit f48a01651b1758550c4d3ee65ec726dfa0658780 upstream.

Commit 82460d97 ("drm/i915: Rework ppgtt init to no require an aliasing
ppgtt") introduced a regression on Broadwell, triggering the following
IOMMU fault at startup:

  vgaarb: device changed decodes: PCI:0000:00:02.0,olddecodes=io+mem,decodes=io+mem:owns=io+mem
  dmar: DRHD: handling fault status reg 2
  dmar: DMAR:[DMA Write] Request device [00:02.0] fault addr 880000
  DMAR:[fault reason 23] Unknown
  fbcon: inteldrmfb (fb0) is primary device

Further commentary from Daniel:

I sugggested this change to David after staring at the offending patch
for a while. I have no idea and theory whatsoever why this would upset
the gpu less than the other way round. But it seems to work. David
promised to chase hw people a bit more to get a more meaningful answer.

Wrt the comment that this deletes: I've done some digging and afaict
loading context before ppgtt enable was once required before our recent
restructuring of the context/ppgtt init code: Before that context sw
setup (i.e. allocating the default context) and hw setup was smashed
together.  Also the setup of the default context was the bit that
actually allocated the aliasing ppgtt structures. Which is the reason
for the context before ppgtt depency.

Or was, since with all the untangling there's no no real depency any
more (functional, who knows what the hw is doing), so the comment is
just stale.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>