xwayland: Fall back to gbm_bo_create if no modifiers are available.
As of Mesa commit 4e3a7dcf6ee4946c46ae8b35e7883a49859ef6fb (gallium: enable EGL_EXT_image_dma_buf_import_modifiers unconditionally) Xwayland uses the gbm_bo_create_with_modifiers even if there are no modifiers available. Fall back to the old case where usage flags are provided if modifiers are not available.
Signed-off-by: David Riley davidriley@chromium.org