OMAP powerdomain: remove pwrdm_clk_state_switch

Nothing calls pwrdm_clk_state_switch(), and the function that seems to be
its ideal use case calls pwrdm_clkdm_state_switch(clk->clkdm), so remove it.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <tero.kristo@nokia.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>