msm: mdss: handle get buffer failure gracefully

If unable to map a buffer, set the pipe in a non-buffer mode which would
cause it to go to a black solid fill. This is better than fetching from
a stale address or a buffer with a different frame configuration.

Change-Id: Ia5b281e695a096c885a2eae0f96126cd3b4da828
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>