drm/i915/selftests: Move igt_atomic_section[] out of the header

Move the definition of the igt_atomic_section[] into a C file, leaving
the declaration in the header so as not to upset headertest!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103104516.1757103-1-chris@chris-wilson.co.uk