staging: sm750fb: use BIT macro for PANEL_DISPLAY_CTRL single-bit fields

Replace complex definition of PANEL_DISPLAY_CTRL register fields with BIT()
macro 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>