OMAPDSS: DSI: dsi_runtime_get/put in pll_init

When DPI uses the DSI PLL for pixel clock, the DPI code will call
dsi_runtime_get/put to keep the DSI block enabled. A much simpler way to
handle this is to do dsi_runtime_get/put in DSI's dsi_pll_init() and
dsi_pll_uninit(), thus removing the need for DSI to call the runtime PM
functions.

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