drm/msm/dpu: Remove unneeded checks in dpu_crtc.c

Removes impossible checks in dpu_crtc.c.
Variable assignments are moved up to be initializations where
possible. Some variables are no longer used, these are removed.

v3: reverted back to original patch

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.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>