drm/msm/dpu: Replace dpu_crtc_reset by atomic helper

Since we removed all suspend logic from the crtc code (see patch 3/4),
dpu_crtc_reset does the same things as drm_atomic_helper_crtc_reset, so let's
just replace it with a call to the atomic helper.

v3: added patch to patchset

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Bruce Wang <bzwang@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>