drm/vkms: WARN when hrtimer_forward_now fails

Add a warn to verify the hrtimer_forward_now return and changes
ret_overrun from int to u64 to match the return value provided by
hrtimer_forward_now.

Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190206200813.d5w7gjpepoeeadiy@smtp.gmail.com