drm/nouveau: change base917c_format from global to static

drm/nouveau: change base917c_format from global to static

Smatch reports this issue
base917c.c:26:1: warning: symbol 'base917c_format'
  was not declared. Should it be static?

base917c_format is only used in base917.c.  Single
file variables should not be global so change
base917c_format'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/20220418141842.296386-1-trix@redhat.com