drm/amdgpu: update vram_info structure in atomfirmware.h

atomfirmware has structure changes in varm_info. Updated it
to the latest one.

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