drm/shmob: Fix return value check in shmob_drm_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: 8f1597c8 ("drm: shmobile: Perform initialization/cleanup at probe/remove time")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>