drm/mgag200: add in missing { } around if block

There is an if block that is missing the { } curly brackets. Add
these in.

Addresses-Coverity: ("Structurally dead code")
Fixes: 94dc57b1 ("drm/mgag200: Rewrite cursor handling")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614143911.21806-1-colin.king@canonical.com