drm/i915: Remove implied length of 2 from GFX_OP_PIPE_CONTROL #define.

Not all PIPE_CONTROLs have a length of 2, so remove it from the #define
and make each invocation specify the desired length.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: implement style suggestion from Ben Widawsdy]
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>