drm/i915: Fix PIPE_CONTROL command on Sandybridge

Sandybridge(Gen6) has new format for PIPE_CONTROL command,
the flush and post-op control are in dword 1 now. This
changes command length field for difference between Ironlake
and Sandybridge.

I tried to test this with noop request and issue PIPE_CONTROL
command for each sequence and track notify interrupts, which
seems work fine. Hopefully we don't need workaround like on
Ironlake for Sandybridge.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>