cogl/renderer: Set error in dmabuf constructor fallback

When the CoglRenderer didn't set the DMA buffer constructor vfunc, we
return NULL. What we didn't do was set the error, meaning the caller
would crash if it tried to look up why DMA buffer construction failed.

!1430
6 jobs for !1430 with wip/cogl-dmabuf-fallback-error in 10 minutes and 29 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Review
passed #900837
check-commit-log

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  Build
passed #900838
build-mutter

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passed #900840
build-without-native-backend-and-wayland

00:01:43

passed #900839
build-without-opengl-and-glx

00:02:30

 
  Test
passed #900842
can-build-gnome-shell

00:02:26

passed #900841
test-mutter

00:06:53