drm/i915: Add has_64k_pages flag

Add a new platform flag, has_64k_pages, to mark the requirement of 64K
GTT page sizes or larger for device local memory access.

Also implies that we require or at least support the compact PT layout
for the ppGTT when using 64K GTT pages.

v2: More explanation for the flag [Thomas]

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208141613.7251-2-ramalingam.c@intel.com