staging: sm750fb: merge reserved bits of PANEL/CRT_DISPLAY_CTRL registers

Use single mask for reserved bits in PANEL_DISPLAY_CTRL and
CRT_DISPLAY_CTRL registers.

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