clk: msm: mdss: enable pll resources once in prepare

Enable PLL resource only once in prepare. The current driver enables and
disables PLL resources multiple times during the course of prepare
routine.

Change-Id: I9fbf58e7e509dda55d8ec01ec7d305ccf1691f94
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>