drm/nouveau/disp/gv100: make gv100_disp_wimm static

Sparse reports this issue
wimmgv100.c:39:1: warning: symbol 'gv100_disp_wimm' was not declared. Should it be static?

This variable is only used in wimmgv100.c.  Single file variables should be static.
So use static as its storage-class specifier.

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/20220425130050.1643103-1-trix@redhat.com