drm/i915: Recompute WM when the cursor size changes

If the cursor width is changed, we may need to recompute our WM to
prevent untold flickering. We hope that the registers are flushed on the
same vblank to prevent underruns...

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>