drivers: gpu: Mark functions as static in vmwgfx_fence.c

Mark functions as static because they are not used outside the file
drm/vmwgfx/vmwgfx_fence.c.

This eliminates the following warnings in drm/vmwgfx/vmwgfx_fence.c:
drivers/gpu/drm/vmwgfx/vmwgfx_fence.c:274:6: warning: no previous prototype for ‘vmw_fences_perform_actions’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_fence.c:900:6: warning: no previous prototype for ‘vmw_fence_obj_add_action’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_fence.c:996:5: warning: no previous prototype for ‘vmw_event_fence_action_create’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>