drm/i915: Plug memory leak in intel_shared_dpll_start_config()

The cleanup path would reset pll->new_config to NULL but wouldn't free
the allocated memory.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>