drm/i915: Setup all page directories for gen8

drm/i915: Setup all page directories for gen8

If the requested size is less than what the full range
of pdps can address, we end up setting pdps for only the
requested area.

The logical context however needs all pdp entries to be valid.
Prior to commit 06fda602dbca ("drm/i915: Create page table allocators")
we have been writing pdp entries with dma address of zero instead
of valid pdps. This is supposedly bad even if those pdps are not
addressed.

As commit 06fda602dbca ("drm/i915: Create page table allocators")
introduced more dynamic structure for pdps, we ended up oopsing
when we populated the lrc context. Analyzing this oops revealed
the fact that we have not been writing valid pdps with bsw, as
it is doing the ppgtt init with 2GB limit in some cases.

We should do the right thing and setup the non addressable part
pdps/pde/pte to scratch page through the minimal structure by
having just pdp with pde entries pointing to same page with
pte entries pointing to scratch page.

But instead of going through that trouble, setup all the pdps
through individual pd pages and pt entries, even for non
addressable parts. And let the clear range point them to scratch
page. This way we populate the lrc with valid pdps and wait
for dynamic page allocation work to land, and do the heavy lifting
for truncating page table tree according to usage.

The regression of oopsing in init was introduced by
commit 06fda602dbca ("drm/i915: Create page table allocators")

v2: Clear the range for the unused part also (Ville)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89350
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Valtteri Rantala <valtteri.rantala@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>