In order to ensure seqno/irq coherency, we currently read a ring register.
The mmio transaction following the interrupt delays the inspection of
the seqno long enough for the MI_STORE_DWORD_IMM to update the CPU
cache. However, it is only the memory timing that is important for the
purposes of the delay, we do not need nor desire the extra forcewake.

v3: Update commentary
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> [v2]
Link: http://patchwork.freedesktop.org/patch/msgid/1460195877-20520-1-git-send-email-chris@chris-wilson.co.uk