drm/i915: Move the priotree struct to its own headers

Over time the priotree has grown from a sorted list to a more
complicated structure for propagating constraints along the dependency
chain to try and resolve priority inversion. Start to segregate this
information from the rest of the request/fence tracking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180418184052.7129-1-chris@chris-wilson.co.uk