msm: mdss: clear irq if RD_PTR intr is detected before clk disable

Before disabling the RD_PTR ISR and turning off the clks, there is a
possibility that some other CPU might have received a READ_PTR ISR few
microseconds before and started to process it. By the time, the ISR
reaches the register read instructions, the clks might be disabled.
To avoid such cases, check for the intr status after disabling the ISR
and clear the irq before disabling the clks.

Change-Id: If9f0e2df22ec7ea26d62b08c9364c2047dda8c51
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>