video: stm32: dcmipp: add set/get_selection handling

Add possibility to perform crop on all pipes and compose (downscale) on
pixel pipes (endpoint #1 and endpoint #2).
Rework the code in order to move the downscale control from
the set_fmt into the set_selection (compose).

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>