drm/i915: Remove select to deleted STOP_MACHINE from Kconfig

Commit 5bab6f60 ("drm/i915: Serialise updates to GGTT with access
through GGTT on Braswell") depended upon a working stop_machine() and
so forced the selection of STOP_MACHINE. However, commit 86fffe4a


("kernel: remove stop_machine() Kconfig dependency") removed the option
STOP_MACHINE from init/Kconfig and ensured that stop_machine()
universally works. Due to the order in which the patches were applied,
removing the select from DRM_I915 got lost during merging.

Remove the now obsolete select statement.

Signed-off-by: Andreas Ziegler <andreas.ziegler@fau.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1453722079-2604-1-git-send-email-andreas.ziegler@fau.de
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>