drm/ttm: fix resource manager size type and description

Leave the man->size units as driver defined.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214093439.2989-1-christian.koenig@amd.com


Reviewed-by: Matthew Auld <matthew.auld@intel.com>