omapdss: use devm_clk_get()

Use devm_clk_get() instead of clk_get() for dss, and for outputs hdmi
and venc. This reduces code and simplifies error handling.

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