drm/i915: Replace ironlake_compute_wm0 with g4x_compute_wm0

The computation of the first-level watermarks for g4x and gen5+ are
based on the same algorithm, so we can refactor those code paths to
use a single function.

Note that g4x_compute_wm0 takes a 'plane' argument while
ironlake_compute_wm0 took a 'pipe' argument. Both should have used a
'plane' argument, so this patch fixes that as well (not that it caused
a problem; ironlake always uses pipe == plane).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>