i915: Don't run retire work handler while suspended

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>