layers : gh682 move gipa dispatch table init

layers : gh682 move gipa dispatch table init

Move the GIPA setup of the dispatch tables for layers into
the layer_init_device_dispatch_table call since we're already
doing a majority of it there.  This removes the need to separately
setup the WSI extension entry-points.

Additionally, memset the table to 0, just to make sure anything
that gets added in the future is obvoiusly not set.

Change-Id: I63c7c107cd9f7957e2766fdbeb8a06bc1ae0eda6