drm/amd/pm/inc/pp_thermal: Mark 'SMU7Thermal{WithDelay}Policy' as __maybe_unused

They are used by some source files which include pp_thermal.h, but not all.

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

 drivers/gpu/drm/amd/amdgpu/../pm/inc/pp_thermal.h:28:41: warning: ‘SMU7ThermalWithDelayPolicy’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../pm/inc/pp_thermal.h:28:41: warning: ‘SMU7ThermalWithDelayPolicy’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../pm/inc/pp_thermal.h:34:41: warning: ‘SMU7ThermalPolicy’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../pm/inc/pp_thermal.h:34:41: warning: ‘SMU7ThermalPolicy’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/../pm/inc/pp_thermal.h:34:41: warning: ‘SMU7ThermalPolicy’ defined but not used [-Wunused-const-variable=]

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: Evan Quan <evan.quan@amd.com>
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>