drm/amd/amdgpu: Correct VRAM width for APUs with GMC9

DDR4 has a 64-bit width not 128-bits.  It was reporting
twice the width.  Tested with my Ryzen 2400G.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>