drm/vmwgfx: remove unused vmw_overlay function

clang with W=1 reports
drivers/gpu/drm/vmwgfx/vmwgfx_overlay.c:56:35: error:
  unused function 'vmw_overlay' [-Werror,-Wunused-function]
static inline struct vmw_overlay *vmw_overlay(struct drm_device *dev)
                                  ^
This function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230321182414.1826372-1-trix@redhat.com