fbdev: mx3fb: avoid warning about psABI change

The arm64 gcc-9 release warns about a change in the calling
conventions:

drivers/video/fbdev/mx3fb.c: In function 'sdc_init_panel':
drivers/video/fbdev/mx3fb.c:506:12: note: parameter passing for argument of type 'struct ipu_di_signal_cfg' changed in GCC 9.1
  506 | static int sdc_init_panel(struct mx3fb_data *mx3fb, enum ipu_panel panel,
      |            ^~~~~~~~~~~~~~
drivers/video/fbdev/mx3fb.c: In function '__set_par':
drivers/video/fbdev/mx3fb.c:848:7: note: parameter passing for argument of type 'struct ipu_di_signal_cfg' changed in GCC 9.1

Change the file to just pass the struct by reference, which is
unambiguous and avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408162551.3928330-1-arnd@arndb.de