gpu: drm: amd: amdgpu: amdgpu: Mark global variables as __maybe_unused

gpu: drm: amd: amdgpu: amdgpu: Mark global variables as __maybe_unused

These 3 variables are used in *some* sourcefiles which include
amdgpu.h, but not *all*.  This leads to a flurry of build warnings.

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

 from drivers/gpu/drm/amd/amdgpu/amdgpu.h:67,
 drivers/gpu/drm/amd/amdgpu/amdgpu.h:198:19: warning: ‘no_system_mem_limit’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/amdgpu.h:197:19: warning: ‘debug_evictions’ defined but not used [-Wunused-const-variable=]
 drivers/gpu/drm/amd/amdgpu/amdgpu.h:196:18: warning: ‘sched_policy’ defined but not used [-Wunused-const-variable=]

 NB: Repeats ~650 times - snipped for brevity.

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: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>