drm: mali-dp: Clear the config_valid flag before using it in wait_event.

config_valid variable is used to signal the activation of the CVAL
request when the vsync interrupt has fired. malidp_set_and_wait_config_valid()
uses the variable in wait_event_interruptible_timeout without clearing it
first, so the wait is skipped.

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