drm/amd/pm: fix is_dpm_running() run error on 32bit system

v1:
the C type "unsigned long" size is 32bit on 32bit system,
it will cause code logic error, so replace it with "uint64_t".

v2:
remove duplicate cast operation.

Signed-off-by: Kevin <kevin1.wang@amd.com>
Suggest-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Jiansong Chen <Jiansong.Chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>