drm/i915/sprite: Fix mem leak in intel_plane_init()

If we ever hit the default case in the switch statement we'll return
from the function without freeing the memory we just allocated to
'intel_plane' (but that has not been used).

This patch gets rid of the leak by freeing the memory just before we
return.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>