fbdev: fsl-diu-fb: mark wr_reg_wa() static

wr_reg_wa() is not an appropriate name for a global function, and doesn't need
to be global anyway, so mark it static and avoid the warning:

drivers/video/fbdev/fsl-diu-fb.c:493:6: error: no previous prototype for 'wr_reg_wa' [-Werror=missing-prototypes]

Fixes: 0d9dab39 ("powerpc/5121: fsl-diu-fb: fix issue with re-enabling DIU area descriptor")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Helge Deller <deller@gmx.de>