drm/amdgpu: Add GDDR6 in vram_name arrary

For printing vram type.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <Tao.Zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>