msm: mdss: Increase fbmem buf ref count before use

The reference count for fbmem buf is not increased before use,
which means it can be get freed unintentionally when the reference
count is decreased to "0". In this case, there is possibility of
use after free. Ensure that fbmem buf refcount is incremented
before use.

Change-Id: I525d41e5496a1123e53a438b5f78d4da8bc046bd
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>