msm: mdss: avoid failure if primary panel pref is not enabled

If device tree property to set primary panel preference is not
currently there, the entire probing sequence will fail. This prevents
certain debugging configurations. Replace this with a warning instead.

CRs-Fixed: 987761
Change-Id: I01949014d95d62ebf9615a6e49f3d500f66e1dea
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>