drm/i915: Stop storing ctx->user_handle

The user_handle need only be known by userspace for it to lookup the
context via the idr; internally we have no use for it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321140711.11190-3-chris@chris-wilson.co.uk