msm: mdss: validate rotator buffer size in user thread

The rotator buffer validation was in the kernel work
queue thread. If the validation fails, there is no way
to notify this failure. This change moves the validation
in the user thread, which allows user to handle the failure
more gracefully.

Change-Id: I21a18de7d96e705fc36339658a27299e9fc607b6
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>