drm/i915: Also perform gpu reset under execlist mode.

It is found that i915 will not reset gpu under execlist mode when
unload module. that will lead to some issues when unload/load module
with different submission mode. e.g. from execlist mode to ring
buffer mode via loading/unloading i915. Because HW is not in a reset
state and registers are not clean under such condition.

Signed-off-by: Niu,Bing <bing.niu@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>