drm/msm/a6xx: Add missing __always_unused

The __build_asserts() function only exists to have a place to put
build-time asserts.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407010401.rfunrBSx-lkp@intel.com/
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/601606/