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 <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Tao Zhou <[e-mail]>
Cc: Hawking Zhang <[e-mail]>
Cc: Jiansong Chen <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 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 } },