drm/armada: Delete dma_buf->k(un)map implemenation

It's a dummy anyway.

Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-6-daniel.vetter@ffwll.ch