dma-buf: give examples of error codes to use

The dma_fence_set_error() function allows to set an error code on a
dma_fence object before it is signaled.

Document some of the potential error codes drivers should use and
especially what they mean.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240826122541.85663-2-christian.koenig@amd.com