msm_fb: display: Make sysfs read request interruptible

Change blocking sysfs read request to be interruptible and protect
vsync timestamp value to prevent corruption.

Change-Id: I370ced4f271e5f1d2c248bed06131c0623d7223f
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>