drm/i915/ttm: remove node usage in our naming

Now that ttm_resource_manager just returns a generic ttm_resource we
don't need to reference the mm_node stuff anymore which mostly only
makes sense for drm_mm_node. In the next few patches we want switch over
to the ttm_buddy_man which is just another type of ttm_resource so
reflect that in the naming.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616152501.394518-5-matthew.auld@intel.com