drm/i915/dgfx: define llc and snooping behaviour

While we do lack the faster shared LLC, we should still have support
for snooping over PCIe.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201014191937.1266226-11-lucas.demarchi@intel.com