This is all kinds of awkward since we now have to contend with using 64K
GTT pages when mapping anything in LMEM(including the page-tables
themselves).

v2(Ram)
  - Document the ppGTT layout and add a better description for the
    different windows.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218184752.7524-12-ramalingam.c@intel.com