drm/selftests: fix spelling mistake "misssing" -> "missing"

There is a spelling mistake in a literal string, fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911091227.5710-1-colin.king@canonical.com