OMAPDSS: add clk_prepare_enable and clk_disable_unprepare

In preparation of OMAP moving to Common Clk Framework(CCF) change
clk_enable() and clk_disable() calls to clk_prepare_enable() and
clk_disable_unprepare() in omapdss. This can be safely done, as omapdss
never enables or disables clocks in atomic context.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: <linux-fbdev@vger.kernel.org>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Mike Turquette <mturquette@linaro.org>
[tomi.valkeinen@ti.com: updated patch description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>