drm/kselftest: fix spellint typo in test-drm_mm.c

This patch fix a spelling typo in test-drm_mm.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200615121151.1557985-1-standby24x7@gmail.com