ANDROID: drm: kirin: remove wait for VACTIVE IRQ

For each display cycle, the Kirin960 display IP will generate a VACTIVE
interrupt followed by a VBLANK. During a FBIOPAN ioctl, the driver will then
wait for the first one to then wait for the second one. This is an issue when
the CPU load is too low: the wait_event() function might trigger a transition
to a deep sleep state and then, waking up from that state will take too much
time to catch the VBLANK interrupt on time, the difference between those two
interrupts being only 60 us.

  * Ideal case:                   ACT                VBL
                                   +                  +
                                   v                  v
                    ---> wait(ACT) +------> wait(VBL) +-->

  * Our case:                     ACT VBL        ACT VBL
                                   +   +          +   +
                                   v   v          v   v
                    ---> wait(ACT) +------> wait(VBL) +-->

The wait for VACTIVE IRQ can safely be removed: there is no hardware access
performed between the VACTIVE and the VBLANK IRQs.

This behavior has been introduced from 4.11 with the following patch:

    a3fbb53f drm/atomic: Wait for vblank whenever a plane is added to state.

Bug: 146450171
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: I66e276c08f04257135c3d05483ce70c58d5070b6