broadcom/vc4: Fix double-unrefcounting of prsc->next with shadows.

broadcom/vc4: Fix double-unrefcounting of prsc->next with shadows.

When we set up the shadow resource we were copying the original resource
as the template, including its prsc->next field.  When we shadowed the
first YUV plane's resource for linear-to-tiled conversion, we would end up
unbalancing the refcount on the shadow resource's destruction.