drm/msm/dp: Shrink locking area of dp_aux_transfer()

drm/msm/dp: Shrink locking area of dp_aux_transfer()

We don't need to hold the lock to inspect the message we're going to
transfer, and we don't need to clear the busy flag either. Take the lock
later and bail out earlier if conditions aren't met.

Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Abhinav Kumar <abhinavk@codeaurora.org>
Cc: Kuogee Hsieh <khsieh@codeaurora.org>
Cc: aravindh@codeaurora.org
Cc: Sean Paul <sean@poorly.run>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Kuogee Hsieh <khsieh@codeaurora.org>
Link: https://lore.kernel.org/r/20210507212505.1224111-3-swboyd@chromium.org
Signed-off-by: Rob Clark <robdclark@chromium.org>