[PATCH] Tell userspace if scanning out of an imported PRIME buffer is safe.

[raof at ubuntu.com]

My past set of patches fix nouveau, radeon, and amdgpu breaking dma-buf sharing when
trying to scanout of an imported PRIME buffer. Driver-agnostic userspace can't safely
try to scanout of such a buffer, though, because the failure-mode is that subsequent
rendering isn't shown on screen.

This adds a DRM_CAP_PRIME_SCANOUT for userspace to detect when it's safe and sensible
to try and scanout of an imported PRIME buffer.

I know that i915 can do this, and nouveau/radeon/amdgpu can't (at the moment).
I do not know which other drivers might be able to support the DRIVER_PRIME_SCANOUT
feature. Suggestions welcome!