intel: Track known prime buffers for re-use

If the application sends us a file descriptor pointing at a prime
buffer that we've already got, we have to re-use the same bo_gem
structure or chaos will result.

Track the set of all known prime objects and look to see if the kernel
has returned one of those for a new file descriptor.

Also checks for prime buffers in the flink case.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>