drm/i915: consolidate ->num_shared_dplls assignement

In the future this won't be just for pch plls, so move it into the
shared dpll init code.

v2: Bikeshed the uncessary {} away while applying to appease
checkpatch.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>