drm/i915: reference count for i915_hw_contexts

Enabling PPGTT and also the need to track which context was guilty of
gpu hang (arb robustness enabling) have put pressure for struct i915_hw_context
to be more than just a placeholder for hw context state.

In order to track object lifetime properly in a multi peer usage, add reference
counting for i915_hw_context.

v2: track i915_hw_context pointers instead of using ctx_ids
(from Chris Wilson)

v3 (Ben): Get rid of do_release() and handle refcounting more compactly.
(recommended by Chis)

v4: kref_* put inside static inlines (Daniel Vetter)
remove code duplication on freeing context (Chris Wilson)

v5: idr_remove and ctx->file_priv = NULL in destroy ioctl (Chris)
This actually will cause a problem if one destroys a context and later
refers to the idea of the context (multiple contexts may have the same
id, but only 1 will exist in the idr).

v6: Strip out the request related stuff. Reworded commit message.
Got rid of do_destroy and introduced i915_gem_context_release_handle,
suggested by Chris Wilson.

v7: idr_remove can't be called inside idr_for_each (Chris Wilson)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v5)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com> (v7)
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Squash sob lines, the patch ping-ponged between Ben and Mika
a bit ...]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>