OMAPDSS: DSI: wait for hsdiv clocks when enabling PLL

At the moment we have two functions to wait for the HSDIV clocks to get
active, dsi_wait_pll_hsdiv_dispc_active and
dsi_wait_pll_hsdiv_dsi_active. Instead of such inconvenient functions,
let's just make sure that the hsdiv clocks are active after the pll has
been enabled.

This patch adds code to dsi_pll_set_clock_div() to wait until HSDIV
clocks are active.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>