drm: radeon: fix overflow on 32bit systems

the type mem->start is unsigned long, so this can overflow on
32bit system, since the type addr is uint64_t.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Young Xiao <YangX92@hotmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>