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 <davidriley@chromium.org>
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