drm/amd/pm: fix warning using uninitialized value of max_vid_step

[ Upstream commit 17e3bea6 ]

Check the return of pp_atomfwctrl_get_Voltage_table_v4
as it may fail to initialize max_vid_step
V2: change the check condition (Tim Huang)

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>