Yes, intel lacks a remapping mechanism to support BAR smaller than the total VRAM size.
Using an Intel discrete GPU on a system that lacks ReBAR support results in somewhat lower performance, not a complete inability to use more than 256MB of VRAM.
alt Hacker News
Using an Intel discrete GPU on a system that lacks ReBAR support results in somewhat lower performance, not a complete inability to use more than 256MB of VRAM.