msm: camera: mem: Remove lock from get_cpu_buf unless mapping

This change removes mutex_lock/unlock from get_cpu_buf function
unless mapping is required. This helps to remove lock from most
calls to this function. There are no checks anyway to see if
buffer is still in use or not before release. The alloc/map/release
calls have a mutex lock when modifying the different parameters
of the bufq struct. Having a mutex lock in this function does not
resolve anything. So removing this helps to be able to call this
function from tasklets as long as mapping is already done.

Change-Id: I48b195ad5c948ce991d8926538d6f61c2e0eef59
Signed-off-by: Harsh Shah <harshs@codeaurora.org>