drm/msm: mdp4: drop vblank get/put from prepare/complete_commit

drm/msm: mdp4: drop vblank get/put from prepare/complete_commit

msm_atomic is doing vblank get/put's already,
currently there no need to duplicate the effort in MDP4

Fix warning:
...
WARNING: CPU: 3 PID: 79 at drivers/gpu/drm/drm_vblank.c:1194 drm_vblank_put+0x1cc/0x1d4
...
and multiple vblank time-outs:
...
msm 5100000.mdp: vblank time out, crtc=1
...

Tested on Nexus 7 2013 (deb), LTS 5.10.50.

Introduced by: 119ecb7fd3b5 ("drm/msm/mdp4: request vblank during modeset")

Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20210715060925.7880-1-david@ixit.cz
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>