drm/i915/selftests: Fixup recursive MI_BB_START for gen3

drm/i915/selftests: Fixup recursive MI_BB_START for gen3

There's no magic bit0 in MI_BB_START for gen3, it's the same dword length
parameter as elsewhere and needs to be zero.

v2: Same bug in both live_requests and live_hanghcheck.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107132
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705154756.5533-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>