drm/amd: Mark IP_BASE definition as __maybe_unused

Silences 166 compile-time warnings like:

 warning: 'UVD0_BASE' defined but not used [-Wunused-const-variable=]
 129 | static const struct IP_BASE UVD0_BASE ={ { { { 0x00007800, 0x00007E00, 0, 0, 0 } },
     |                             ^~~~~~~~~
 warning: 'UMC0_BASE' defined but not used [-Wunused-const-variable=]
 123 | static const struct IP_BASE UMC0_BASE ={ { { { 0x00014000, 0, 0, 0, 0 } },
     |                             ^~~~~~~~~

Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>