amd/addrlib: mark microBlockDim as MAYBE_UNUSED in Addr::V2::Gfx9Lib::HwlComputeBlock...

amd/addrlib: mark microBlockDim as MAYBE_UNUSED in Addr::V2::Gfx9Lib::HwlComputeBlock256Equation

Only used, when asserts are enabled.

Fixes an unused-but-set-variable warning with GCC 8:
 ../../../src/amd/addrlib/gfx9/gfx9addrlib.cpp: In member function 'virtual ADDR_E_RETURNCODE Addr::V2::Gfx9Lib::HwlComputeBlock256Equation(AddrResourceType, AddrSwizzleMode, unsigned int, ADDR_EQUATION*) const':
 ../../../src/amd/addrlib/gfx9/gfx9addrlib.cpp:2473:15: warning: variable 'microBlockDim' set but not used [-Wunused-but-set-variable]
          Dim2d microBlockDim = Block256_2d[elementBytesLog2];
                ^~~~~~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>