kmsgrab: Use GetFB2 if available

kmsgrab: Use GetFB2 if available

The most useful feature here is the ability to automatically extract the
framebuffer format and modifiers.  It also makes support for multi-plane
framebuffers possible, though none are added to the format table in this
patch.

This requires libdrm 2.4.101 (from April 2020) to build, so it includes a
configure check to allow compatibility with existing distributions.  Even
with libdrm support, it still won't do anything at runtime if you are
running Linux < 5.7 (before June 2020).