drm/i915: fix module unload after context merge

drm/i915: fix module unload after context merge

commit c1e15fa1b487138513c1f63a441b7e90bf334a17
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Mon Jun 4 14:42:56 2012 -0700

    drm/i915: reset the GPU on context fini

broke module unload because it reset the gpu before we've stopped
touching it. Later on in the unload sequence the ringbuffer code
complained that the gpu would idle properly (because intel_gpu_reset
only resets the hw and not our sw state).

v2: Reorder things so that we reset the gpu _before_ we release the
backing storage of the default context.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51183
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>