drm/i915: Compute WM for current cursor size

Now that we can use different cursor size, we can not hardcode 64 pixels
as the cursor width anymore.

v2: Apply to 965gm/g4x paths as well

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>