drm/i915: Rename INTEL_REGION_LMEM with INTEL_REGION_LMEM_0

With the upcoming multitile support each tile will have its own
local memory. Mark the current LMEM with the suffix '0' to
emphasise that it belongs to the root tile.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318233938.149744-2-andi.shyti@linux.intel.com