drm/amdkfd: Fix for possible integer overflow

Fix for possible integer overflow when doing addition.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Yat Sin <david.yatsin@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>