drm/rockchip: fix dma_alloc_attrs() error check

dma_alloc_attrs() returns NULL if it cannot allocate a dma buffer (or
mapping), not a negative error code.

Rerported-by: Pawel Osciak <posciak@chromium.org>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>