msm: mdss: Align the shared memory length to PAGE_SIZE

It is possible that userspace modifies the frame buffer
parameters. In such cases ensure to align the frame
buffer size to avoid mmap failures.

Change-Id: I4a002694c26c8fface45d8e274d79d5624cc8158
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>