drm/amd/pm: fix compile warnings about variable used uninitialized

Fix the compile warnings below:
>> drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/smu7_hwmgr.c:1743:13: warning: variable 'min' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
>> drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/smu7_hwmgr.c:1743:13: warning: variable 'max' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>