drm/i915: Fix pipe enabled mask for pipe C in WM calculations

Fix the incorrect enabled pipes mask for pipe C in the WM calculations.

Additionally, in an effort to make the code easier to understand,
populate the mask with 1 << PIPE_[ABC] instead of raw numbers.

v2: Use 1 << PIPE_[ABC] (ickle/danvet)
v3: Pass PIPE_[ABC] to g4x_compute_wm0() (ickle)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>