drm/amd/powerplay: add feature check in unforce_dpm_levels function (v2)

if not check dpm feature is enabled, it will cause show smc send message
failed log in dmesg log.
eg:
echo "auto" > power_dpm_force_performance_level

v2: whitespace fix (Alex)

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Rui Teng <rui.teng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>