Our static analysis tool noticed that 'reg' could be used uninitialized if
we are trying to get a PLL to drive VGA and SPLL is already in use
(plls->spll_refcoung != 0).

In the (error) case above, let's return false to the caller and emit an
error.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>