msm: mdss: move read ptr irq configuration to the vsync handler

Enable and disable the read pointer interrupt only during
the time that the vsync handlers request this interrupt.
This also makes sure that mdp clocks are enabled during
all this time.

Change-Id: I2564a3b0cf05325c282244f5b60df10d44f5b364
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>