drm/msm/dpu: Remove useless checks in dpu_encoder

A couple more useless checks to remove in dpu_encoder.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210913175747.47456-10-sean@poorly.run #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20210915203834.1439-10-sean@poorly.run #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20211001151145.55916-10-sean@poorly.run #v3

Changes in v2:
-None
Changes in v3:
-None
Changes in v4:
-None
Link: https://lore.kernel.org/r/20211105030434.2828845-10-sean@poorly.run



Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>