drm/amdkfd: Fix possible infinite loop

When the loop predicating timeout parameter passed happens to
not be a multiple of 20 the unsigned integer will overflow and
the loop will become unbounded.

Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>