drm/i915/xelpd: First stab at DPT support

Add support for DPT (display page table). DPT is a
slightly peculiar two level page table scheme used for
tiled scanout buffers (linear uses direct ggtt mapping
still). The plane surface address will point at a page
in the DPT which holds the PTEs for 512 actual pages.
Thus we require 1/512 of the ggttt address space
compared to a direct ggtt mapping.

We create a new DPT address space for each framebuffer and
track two vmas (one for the DPT, another for the ggtt).

TODO:
- Is the i915_address_space approaach sane?
- Maybe don't map the whole DPT to write the PTEs?
- Deal with remapping/rotation? Need to create a
  separate DPT for each remapped/rotated plane I
  guess. Or else we'd need to make the per-fb DPT
  large enough to support potentially several
  remapped/rotated vmas. How large should that be?

Signed-off-by: Ville Syrjälä <[email protected]>
Signed-off-by: Bommu Krishnaiah <[email protected]>
Cc: Wilson Chris P <[email protected]>
Cc: Tang CQ <[email protected]>
Cc: Auld Matthew <[email protected]>
Reviewed-by: Uma Shankar <[email protected]>
Reviewed-by: Wilson Chris P <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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