drm/amd/include/dimgrey_cavefish_ip_offset: Mark top-level IP_BASE as __maybe_unused

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

In file included from drivers/gpu/drm/amd/amdgpu/dimgrey_cavefish_reg_init.c:28:
drivers/gpu/drm/amd/amdgpu/../include/dimgrey_cavefish_ip_offset.h:151:29: warning: ‘UMC_BASE’ defined but not used [-Wunused-const-variable=]
151 | static const struct IP_BASE UMC_BASE = { { { { 0x00014000, 0x02425800, 0, 0, 0, 0 } },
| ^~~~~~~~
drivers/gpu/drm/amd/amdgpu/../include/dimgrey_cavefish_ip_offset.h:81:29: warning: ‘FUSE_BASE’ defined but not used [-Wunused-const-variable=]
81 | static const struct IP_BASE FUSE_BASE = { { { { 0x00017400, 0x02401400, 0, 0, 0, 0 } },
| ^~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../include/dimgrey_cavefish_ip_offset.h:74:29: warning: ‘DPCS_BASE’ defined but not used [-Wunused-const-variable=]
74 | static const struct IP_BASE DPCS_BASE = { { { { 0x00000012, 0x000000C0, 0x000034C0, 0x00009000, 0x02403C00, 0 } },
| ^~~~~~~~~

NB: Snipped lots of these
Acked-by: Christian König <christian.koenig@amd.com>
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: Tao Zhou <tao.zhou1@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Jiansong Chen <Jiansong.Chen@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>

drivers/gpu/drm/amd/include/dimgrey_cavefish_ip_offset.h

@@ -33,7 +33,7 @@ struct IP_BASE_INSTANCE
 struct IP_BASE
 {
     struct IP_BASE_INSTANCE instance[MAX_INSTANCE];
-};
+} __maybe_unused;
 
 
 static const struct IP_BASE ATHUB_BASE = { { { { 0x00000C00, 0x02408C00, 0, 0, 0, 0 } },