drm/amdgpu: fix compiler warning

Fixes below warning:
drivers/gpu/drm/amd/amdgpu/df_v3_6.c: In function ‘df_v3_6_reset_perfmon_cntr’:
drivers/gpu/drm/amd/amdgpu/df_v3_6.c:571:2: warning: ‘hi_base_addr’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
  571 |  df_v3_6_perfmon_wreg(adev, lo_base_addr, 0, hi_base_addr, 0);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>