compositor-drm: Remove addfb warning for user buffers

THe KMS AddFB call can fail for any reason at all: format/modifier not
suitable, stride not aligned, allocation not contiguous, etc. If this
happens with Weston's own buffers, the result is bad - no composition
output.

Failing AddFB from user-supplied buffers though, is not an error. The
user can't necessarily allocate suitable buffers, nor does it have to.
Don't spam the log with warnings when we fail on user buffers.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>