drm/pl111: Fix return value check in pl111_amba_probe()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: bed41005 ("drm/pl111: Initial drm/kms driver for pl111")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170521010152.6186-1-weiyj.lk@gmail.com