drm/i915/gem: Check that the context wasn't closed during setup

drm/i915/gem: Check that the context wasn't closed during setup

As setup takes a long time, the user may close the context during the
construction of the execbuf. In order to make sure we correctly track
all outstanding work with non-persistent contexts, we need to serialise
the submission with the context closure and mop up any leaks.

Signed-off-by: Chris Wilson <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]