Revert "drm/dsi: Fix byte order of DCS set/get brightness"

The commit 8a744ca6 ("ANDROID: drm/dsi: Fix byte order of DCS
set/get brightness") modified the upstream API and this makes us
hard to reconcile it to android-4.19 without breaking APIs for GKI.
Rollback the upstream API and implement it in the downstream side.

Bug: 156865857
Change-Id: I0f11aec2249d305dbed77b05b197f67ffbfb00dd
Signed-off-by: Ken Huang <kenbshuang@google.com>
Signed-off-by: Chris Lu <luchris@google.com>
Signed-off-by: Will Mcvicker <willmcvicker@google.com>