gpio: omap_gpio: Fix unused function warning in non-DM case

In the case of non-DM_GPIO the function get_gpio_index() will never be
called, and clang will warn about this.  Move this to be with the other
non-DM code for easier removal later.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>