amd/addrlib: mark *pEqToCheck as MAYBE_UNUSED in Addr::V2::Gfx9Lib::ComputeStereoInfo...

amd/addrlib: mark *pEqToCheck as MAYBE_UNUSED in Addr::V2::Gfx9Lib::ComputeStereoInfo (v2)

Only used, when asserts are enabled.

Fixes an unused-variable warning with GCC 8:
 ../../../src/amd/addrlib/gfx9/gfx9addrlib.cpp: In member function 'ADDR_E_RETURNCODE Addr::V2::Gfx9Lib::ComputeStereoInfo(const ADDR2_COMPUTE_SURFACE_INFO_INPUT*, ADDR2_COMPUTE_SURFACE_INFO_OUTPUT*, unsigned int*) const':
 ../../../src/amd/addrlib/gfx9/gfx9addrlib.cpp:3879:34: warning: unused variable 'pEqToCheck' [-Wunused-variable]
              const ADDR_EQUATION *pEqToCheck        = &m_equationTable[eqIndex];
                                   ^~~~~~~~~~

v2: Don't realign other variable definitions, to keep in line with file
    style (Marek)

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