drm/i915/ringbuffer: Move irq seqno barrier to the GPU for gen5

The irq_seqno_barrier is a tradeoff between doing work on every request
(on the GPU) and doing work after every interrupt (on the CPU). We
presume we have many more requests than interrupts! However, for
Ironlake, the workaround is a pretty hideous usleep() and so even though
it was found we need to repeat the MI_STORE_DWORD_IMM 8 times, or about
1us of GPU time, doing so is preferrable than requiring a sleep of
125-250us on the CPU where we desire to respond immediately (ideally from
within the interrupt handler)!

The additional MI_STORE_DWORD_IMM also have the side-effect of flushing
MI operations from userspace which are not caught by MI_FLUSH!

Testcase: igt/gem_sync
Testcase: igt/gem_exec_whisper
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181228171641.16531-5-chris@chris-wilson.co.uk