drm: mali-dp: Clear CVAL when leaving config mode

It's possible for CVAL to get set whilst we are in config mode. If this
happens, afer we leave config mode the HW will latch whatever
configuration is in the registers at the next vsync. Most likely this
will be a partial configuration, as we'll be racing against the ongoing
atomic_commit.

To avoid this, clear CVAL before leaving config mode.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>