amd/addrlib: mark physicalSliceSize as MAYBE_UNUSED in Addr::V1::EgBasedLib::HwlGetSi...

amd/addrlib: mark physicalSliceSize as MAYBE_UNUSED in Addr::V1::EgBasedLib::HwlGetSizeAdjustmentMicroTiled

Only used, when asserts are enabled.

Fixes an unused-but-set-variable warning with GCC 8:
 ../../../src/amd/addrlib/r800/egbaddrlib.cpp: In member function 'virtual long long unsigned int Addr::V1::EgBasedLib::HwlGetSizeAdjustmentMicroTiled(unsigned int, unsigned int, ADDR_SURFACE_FLAGS, unsigned int, unsigned int, unsigned int, unsigned int*, unsigned int*) const':
 ../../../src/amd/addrlib/r800/egbaddrlib.cpp:4111:13: warning: variable 'physicalSliceSize' set but not used [-Wunused-but-set-variable]
      UINT_64 physicalSliceSize;
              ^~~~~~~~~~~~~~~~~

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