Changing the order switchable filter enum constants.

This changeset allows to remove vp9_switchable_interp and
vp9_switchable_interp_map arrays and make code much clear. Actually we
still have to use these mapping but only inside read_interp_filter_type and
write_interp_filter_type functions.

Change-Id: I4026c6f8c4acefba6c81421b7bacbaa52cc45f50