drm/armada: remove some dead code

'dma_buf_map_attachment()' can not return NULL, so there is no need to
check for it.

Also add a space in order to improve layout.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>