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

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

This eliminates the following warnings in drm/vmwgfx/vmwgfx_kms.c:
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:43:6: warning: no previous prototype for ‘vmw_clip_cliprects’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:426:6: warning: no previous prototype for ‘vmw_framebuffer_surface_destroy’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:592:5: warning: no previous prototype for ‘vmw_framebuffer_surface_dirty’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:757:6: warning: no previous prototype for ‘vmw_framebuffer_dmabuf_destroy’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:943:5: warning: no previous prototype for ‘vmw_framebuffer_dmabuf_dirty’ [-Wmissing-prototypes]
drivers/gpu/drm/vmwgfx/vmwgfx_kms.c:1666:5: warning: no previous prototype for ‘vmw_du_update_layout’ [-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>