drm/kms: fix spelling in error message

Signed-off-by: Pavel Roskin <proski@gnu.org>
[clemens@ladisch.de: merged into drm_fb_helper]
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>