drm/i915: Serialise access to GFX_FLSH_CNTL

Now that many threads may try to use the same mmio to flush the global
buffers after updating the PTE, serialise access to the mmio to prevent
concurrent access on gen7.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191130162320.1683424-1-chris@chris-wilson.co.uk