backend-drm: Retrieve reason if dmabuf import failed

As we could have situations where dmabuf import failed when attempting
to figure it the framebuffer is scanout-capable, make sure we also have
a way to store that information. Otherwise, we could end up
NULL-dereferencing, as we don't provide a valid storage for it.

Further more, with this, we also print out the reason why it failed, to
aid in further debugging.

Observed on platforms where GBM_BO_HANDLE failed + in combination w/
direct-display proto extension.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>