drm/i915: Initialise g4x watermarks for disabled pipes

We were using uninitialised watermarks values for disabled pipes which
were combined into a single WM register and so corrupting the values for
the enabled pipe and upsetting the display hardware.

Reported-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=32612
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>