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

Removes some checks from dpu_plane.c that will never result in an error.
Subsequent variable assignments become part of the initialization wherever
possible. Unused variables are removed.

v3: removed additional impossible checks and called helper function
_dpu_plane_get_kms() where possible.

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>