drm/msm: dpu: Remove impossible checks

This patch removes some checks which are impossible to hit. As a result,
we can move some of the local var assignments into the declarations.

Changes in v2:
- None

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>