drm/i915: Assign every HW context a unique ID

The hardware tracks contexts and expects all live contexts (those active
on the hardware) to have a unique identifier. This is used by the
hardware to assign pagefaults and the like to a particular context.

v2: Reorder to make sure ctx->link is not left dangling if the
assignment of a hw_id fails (Mika).

v3: We have 21bits of context space, not 20.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-17-git-send-email-chris@chris-wilson.co.uk