This commit adds a filter callback, which can be used to filter
out interval RB nodes matching a certain interval down to a
single one.

This is needed for the upcoming SDMA-side caching where buffers
will need to be filtered by their virtual address.
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>