drm/amd/powerplay: fix a crash when overclocking Vega M

Avoid kernel crash when vddci_control is SMU7_VOLTAGE_CONTROL_NONE and
vddci_voltage_table is empty. It has been tested on Intel Hades Canyon
(i7-8809G).

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=208489
Fixes: ac7822b0

 ("drm/amd/powerplay: add smumgr support for VEGAM (v2)")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Qiu Wenbo <qiuwenbo@phytium.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org