drm/i915: use PIPE_CONTROL instruction on Ironlake and Sandy Bridge

Since 965, the hardware has supported the PIPE_CONTROL command, which
provides fine grained GPU cache flushing control.  On recent chipsets,
this instruction is required for reliable interrupt and sequence number
reporting in the driver.

So add support for this instruction, including workarounds, on Ironlake
and Sandy Bridge hardware.

https://bugs.freedesktop.org/show_bug.cgi?id=27108



Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>