drm/nouveau/disp/gv100: make gv100_disp_wndw and gv100_disp_wndw_mthd static

Sparse reports these issues
wndwgv100.c:120:1: warning: symbol 'gv100_disp_wndw_mthd' was not declared. Should it be static?
wndwgv100.c:140:1: warning: symbol 'gv100_disp_wndw' was not declared. Should it be static?

These variable are only used in wndwgv100.c.  Single file variables should be static.
So use static as their storage-class specifiers.

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/20220425131308.158635-1-trix@redhat.com