Add CONFIG_X86 check to fix the build error.

Fixes: 49aa98ca ("drm/amd/amdgpu: Only reserve vram for firmware with vega9 MS_HYPERV host.")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>