mdss: fb: Unblock blit ioctl calls when commit is occuring

Since BLIT calls have no effect on the frame being commited it
does not need to block in pan_idle.

Change-Id: Ic0cfa7c0c275ddc051b60ff8823fcd7299cb8b57
Signed-off-by: Terence Hampson <thampson@codeaurora.org>