drm: simpledrm: fix a potential NULL dereference

The drm_format_info() function returns NULL if the format is
unsupported, but the simplefb_get_validated_format() is expected to
return error pointers.  If we propagate the NULL return then it will
lead to a NULL dereference in the callers.  Swap the NULL and trade it
in for an ERR_PTR(-EINVAL).

Fixes: 11e8f5fd ("drm: Add simpledrm driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/YJ+aC47XX58ICXax@mwanda