drm/st7571-i2c: Fix IS_ERR() vs NULL checks in probe()

drm/st7571-i2c: Fix IS_ERR() vs NULL checks in probe()

The devm_kzalloc() function returns NULL on failure, not error pointers.
Also printing an error message for kmalloc() failures is against kernel
style so just return -ENOMEM without printing a message.  (Kmalloc
already prints a message).

Fixes: 4b35f0f41ee2 ("drm/st7571-i2c: add support for Sitronix ST7571 LCD controller")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/aBHZYgPPPYY-J8Vd@stanley.mountain
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>