msm: mdss: align frame buffer size to PAGE_SIZE

When updating userspace with the shared memory length,
the size of each buffer needs to be 4k aligned for
performance. This is also required to avoid mmap failures
in the case of recovery and other usecases where the
requested size is 4k aligned.

Change-Id: I6e97f9c76ea1aff07572c500f30f888b6e12f9b4
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>