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.