drm/sitronix/st7571-i2c: Make st7571_panel_data variables static const

The kernel test robot reported that sparse gives the following warnings:

make C=2 M=drivers/gpu/drm/sitronix/
CC [M] st7571-i2c.o
CHECK st7571-i2c.c
st7571-i2c.c:1027:26: warning: symbol 'st7567_config' was not declared. Should it be static?
st7571-i2c.c:1039:26: warning: symbol 'st7571_config' was not declared. Should it be static?
MODPOST Module.symvers
LD [M] st7571-i2c.ko

Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Javier Martinez Canillas <[e-mail]>

drivers/gpu/drm/sitronix/st7571-i2c.c

···
 	drm_dev_unplug(&st7571->dev);
 }
 
-struct st7571_panel_data st7567_config = {
+static const struct st7571_panel_data st7567_config = {
 	.init = st7567_lcd_init,
 	.parse_dt = st7567_parse_dt,
 	.constraints = {
···
 	},
 };
 
-struct st7571_panel_data st7571_config = {
+static const struct st7571_panel_data st7571_config = {
 	.init = st7571_lcd_init,
 	.parse_dt = st7571_parse_dt,
 	.constraints = {