drm/xe: Use NULL PTEs as scratch PTEs

Currently scratch PTEs are write-enabled and points to a single scratch
page. This has the side effect that buggy applications with out-of-bounds
memory accesses may not notice the bad access since what's written may
be read back.

Instead use NULL PTEs as scratch PTEs. These always return 0 when reading,
and writing has no effect. As a slight benefit, we can also use huge NULL
PTEs.

One drawback pointed out is that debugging may be hampered since previously
when inspecting the content of the scratch page, it might be possible to
detect writes to out-of-bound addresses and possibly also
from where the out-of-bounds address originated. However since the scratch
page-table structure is kept, it will be easy to add back the single
RW-enabled scratch page under a debug define if needed.

Also update the kerneldoc accordingly and move the function to create the
scratch page-tables from xe_pt.c to xe_pt.h since it is accessing
vm structure internals and this also makes it possible to make it static.

v2:
- Don't try to encode scratch PTEs larger than 1GiB.
- Move xe_pt_create_scratch(), Update kerneldoc.
v3:
- Rebase.

Cc: Brian Welty <[email protected]>
Cc: Matt Roper <[email protected]>

Signed-off-by: Thomas Hellström <[email protected]>
Acked-by: Lucas De Marchi <[email protected]> #for general direction.
Reviewed-by: Brian Welty <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Rodrigo Vivi <[email protected]>

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