drm/i915: Add stable memory region names

At the moment memory region names are a bit too varied and too
inconsistent to be used for ABI purposes, like for upcoming fdinfo
memory stats.

System memory can be either system or system-ttm. Local memory has the
instance number appended, others do not. Not only incosistent but thi
kind of implementation detail is uninteresting for intended users of
fdinfo memory stats.

Add a stable name always formed as $type$instance. Could have chosen a
different stable scheme, but I think any consistent and stable scheme
should do just fine.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107101806.608990-5-tvrtko.ursulin@linux.intel.com