staging: sm750fb: change definition of PANEL_DISPLAY_CTRL multi-bit fields

Use more straight-forward definitions for multi-bit fields of
PANEL_DISPLAY_CTRL register and use open-coded implementation for register
manipulation

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