drm/amd/pm/inc/smu_v11_0: Mark 'smu11_thermal_policy' as __maybe_unused

It's used in some, but not all source files which include 'smu_v11_0.h'.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/smu_v11_0.c:36:
 drivers/gpu/drm/amd/amdgpu/../pm/inc/smu_v11_0.h:61:43: warning: ‘smu11_thermal_policy’ defined but not used [-Wunused-const-variable=]
 61 | static const struct smu_temperature_range smu11_thermal_policy[] =
 | ^~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>