dmaengine: Move icg helpers to global header

Now that we can have ICGs set for both the source and destination (using
the icg field of struct data_chunk) or for only the source or the
destination (using the dst_icg or src_icg respectively), and that these
fields can be ignored depending on other parameters (src_inc, src_sgl,
etc.), the logic to get the actual ICG value can be quite tricky.

The XDMAC driver was already implementing it, but since we will need it in
other drivers, we can move it to the main header file.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>