It may be possible, albeit unlikely, to encounter integer overflow
during the multiplication of several unsigned int variables, the
result being assigned to a variable 'size' of wider type.

Prevent this potential behaviour by converting one of the multiples
to unsigned long.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 0242f74d ("drm/radeon: clean up CS functions in r100.c")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>