screen-cast: Enable DMA buffers only for i915

Seems DMA buffer based interprocess buffer sharing is more broken than not, so for now only enable it when using the i915 driver.

For example vmwgfx, qxl and radeon, it results in mmap() failing to mmap the memory region. Other drivers, e.g. amdgpu will function, but may hit very slow memory download paths, resulting in worse performance.