msm: kgsl: Use vma_area_struct with proper locks

Make sure to use proper locks when using the
vma_struct_area. This will avoid the race condition
in a scenario where one thread gets a vma_struct_area
and other thread is unmapping it vma from the process.

Change-Id: I6c7837d1a8dd24fc6955ab5be8b1917a42f2cb53
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>