drm/tve200: Check for IS_ERR instead of NULL in probe

devm_ioremap_resource() returns error pointer, it never returns NULL on
error.

Fixes: 179c02fe ("drm/tve200: Add new driver for TVE200")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170925102520.a7spymwqqbsczzz2@mwanda