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. Without this change a suboptimal buffer might be allocated which could result in extra copies of buffers for scanout. Signed-off-by: David Riley <email@example.com>