disp: msm: do not print error when crtc state is empty

Sometimes committed state doesn't contain any crtcs. For example,
fbdev emulation is still pushing null commits to DRM even if the
cable has been plugged out. Avoid flooding the logs in this case.

Change-Id: I6b7f049b80409c4a2ae057839ede6dd8af10d03b
Signed-off-by: Ray Zhang <rayz@codeaurora.org>