drm/i915/execlists: Clear context-switch interrupt earlier in the reset

Move the clearing of the CS-interrupt into the engine reset phase,
before the current init-hw phase. This helps clarify that we clear the
pending interrupts prior to any restarting of the execlists.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-16-chris@chris-wilson.co.uk