drm/i915: STOP_MACHINE is no more, stop selecting it

drm/i915: STOP_MACHINE is no more, stop selecting it

The STOP_MACHINE kconfig symbol was removed upstream after making
stop_machine() always work, commit 86fffe4a61dd ("kernel: remove
stop_machine() Kconfig dependency"), and was removed from i915's Kconfig
in commit 21fabbebff0e ("drm/i915: Remove select to deleted
STOP_MACHINE from Kconfig").

However, I accidentally reintroduced the select when rebasing an older
commit that also was dependent upon a working stop_machine.

Fixes: 9f267eb8d2ea ("drm/i915: Stop the machine whilst capturing...")
Reported-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161019180635.27459-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>