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

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507180503.nfyD9uRv-lkp@intel.com
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250718152534.729770-1-javierm@redhat.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>