gpu: host1x: fix an integer overflow check

Tegra is a 32 bit arch.  On 32 bit systems then size_t is 32 bits so
"total" will never be higher than UINT_MAX because of integer overflows.
We need cast to u64 first before doing the math.

Also the addition earlier:

        unsigned int num_unpins = num_cmdbufs + num_relocs;

That can overflow as well, but I think it's still safe because we check
both "num_cmdbufs" and "num_relocs" again in this test.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>