drm/mali-dp: Check PM status when sharing interrupt lines

If an instance of Mali DP hardware shares the interrupt line with
another hardware (usually another instance of the Mali DP) its
interrupt handler can get called when the device is suspended.

Check the PM status before making access to the hardware registers
to avoid deadlocks.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>