drm/i915: Flag the execlists context object as dirty after every use

Everytime we use the logical context with execlists it becomes dirty (as
the hardware will write the new register values afterwards, as well as
the GPU state that will be used). We need to then flag the context as
dirty everytime since after a swap-out/swap-in cycle the dirty flag will
be cleared, and a further swap-out cycle will then loose the most recent
GPU state.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>