At leavevt and lastclose time, cancel any pending retire work handler
invocation, and keep the retire work handler from requeuing itself if it is
currently running.

This patch restructures i915_gem_idle to perform all of these tasks instead
of having both leavevt and lastclose call a sequence of functions.
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>