drm/amd/powerplay: tidy up ret checks in amd_powerplay.c (v3)

v2: squash in regression fix (Rex)
v3: Squash in regression fix (Rex)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>