drm/i915/ttm: implement access_memory

It looks like we need this for local-memory, if we want to use ptrace.
Something more is still needed if we want to handle non-mappable memory,
which looks quite annoying.

v2:
  - ttm_bo_kmap doesn't seem to work well here, and seems to expect
    contiguous resource.
v3(Andi):
  - s/PAGE_SIZE/bytes/ when passing in the size of the mapping.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/6989


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003172819.99245-1-matthew.auld@intel.com