drm/mgag200: set variable mgag200_modeset storage-class-specifier to static

smatch reports
drivers/gpu/drm/mgag200/mgag200_drv.c:23:5: warning: symbol
  'mgag200_modeset' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230517134140.874179-1-trix@redhat.com