drm/nouveau/gr/gf100-: change gf108_gr_fwif from global to static

Smatch reports this issue
gf108.c:147:1: warning: symbol 'gf108_gr_fwif'
  was not declared. Should it be static?

gf108_gr_fwif is only used in gf108.c.  Single
file variables should not be global so change
gf108_gr_fwif's storage-class specifier to static.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220418152810.3280502-1-trix@redhat.com