xwayland: don't pass INVALID modifier to EGL

When importing a GBM buffer without an explicit modifier to EGL,
don't pass DRM_FORMAT_MOD_INVALID. Instead, omit the modifier.

Fixes: f1572937 ("Use EGL_LINUX_DMA_BUF_EXT to create GBM bo EGLImages")
4 jobs for egl-import-mod-invalid in 3 minutes and 10 seconds (queued for 1 second)
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