msm: mdss: add support to set mdp clock rate before enable

Current clock driver requires that before enabling
the core clock after power collapse, the set rate
must be called. Ensure this sequence by always calling
the set rate for the mdp core clock before enable.

Change-Id: I872f18235b0b1685dfdee3de99827d031077a012
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>