dmaengine: hdmac: factorise memset descriptor allocation

The memset and scatter gathered memset are going to use some common logic
to create their descriptors.

Move that logic into a function of its own so that we can share it with the
future memset_sg callback.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>