drm/i915: Only update shared dpll state when needed

Instead of every time it isn't active: We only need to do that when
the pll is currently unused, i.e. when pll->refcount == 0. For
paranoia add a warning for the ibx case where plls have a fixed
mapping and hence should always be unused after the call to
intel_put_shared_dpll.

v2: Simplify control flow and use struct assignment instead of memcpy
as suggested by Damien.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>