media: v4l2-mem2mem: extend m2m APIs for more accurate buffer management

this add functions for:
  - remove buffers from src/dst queue by index
  - remove exact buffer from src/dst queue

also extends m2m API to iterate over a list of src/dst buffers
in safely and non-safely manner.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>