OMAPDSS: DSI: always power on hsclk & hsdiv

The DSS PLL has support to power on the PLL's highspeed clock output
and HSDIV output separately. In practice both need to powered on, as in
most OMAP's that's the only working configuration. We already do that in
dsi_pll_init(), by overriding the passed arguments so that both are
always powered.

Simplify the code by removing the support for choosing which outputs to
power on.

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