drm/msm/dsi_pll_10nm: Remove impossible check

While I'm in here, cut this out, pdev can't be NULL

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-2-sean@poorly.run