drm/radeon/ci_dpm: Use int type to store negative error codes

Change the 'ret' variable in ci_populate_all_graphic_levels()
and ci_populate_all_memory_levels() from u32 to int, as it needs to store
either negative error codes or zero returned by other functions.

Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but can be confusing.  Additionally, assigning negative error
codes to unsigned type may trigger a GCC warning when the -Wsign-conversion
flag is enabled.

No effect on runtime.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>