drm/i915: support 64K GTT pages for discrete cards

discrete cards optimise 64K GTT pages for local-memory, since everything
should be allocated at 64K granularity. We say goodbye to sparse
entries, and instead get a compact 256B page-table for 64K pages,
which should be more cache friendly. 4K pages for local-memory
are no longer supported by the HW.

v4: don't return uninitialized err in igt_ppgtt_compact
Reported-by: kernel test robot <lkp@intel.com>

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218184752.7524-8-ramalingam.c@intel.com