intel: add prime interface for getting/setting a prime bo. (v4)

This adds interfaces for the X driver to use to create a
prime handle from a buffer, and create a bo from a handle.

v2: use Chris's suggested naming (well from at least for consistency)
v3: git commit --amend fail
v4: fix as per Chris's suggestions, group assignments, add get tiling

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>