drm/amdgpu: fix getting vram info for gfx12

gfx12 query video mem channel/type/width from umc_info of atom list, so
fix it accordingly.

Signed-off-by: Frank Min <Frank.Min@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>