drm/i915/selftests: Unconditionally do a chipset flush before emit_bb_start

Experience teaches us over and over again that coherency on Baytrail
requires the odd heavy hammer, and in particular clflush alone is not
enough to guarrantee that writes from the CPU are picked up by the CS.
Do as we do elsewhere and ensure we have an unconditional
i915_gem_chipset_flush() after writing to memory and submitting a batch
to HW.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107499


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/20180806144604.8346-1-chris@chris-wilson.co.uk