staging: sm750fb: change definition of PANEL_PLANE_BR fields

Use stratight-forward defintion of PANEL_PLANE_BR register fields and
use open-coded implementation for register manipulations

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>