msm: camera: memmgr: Use dma_buf for kernel allocations

Currently, both user and kernel space use file descriptors
as the primary key when tracking allocated buffers. However,
memory allocated within the kernel may not have any associated
camera process. This makes it difficult to clean up the file
descriptors associated with kernel allocated memory. Hence,
change the primary key of kernel allocated memory to use
dma_buf instead.

Change-Id: I55a65a28d89ca94168e366d441a570039aa08076
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>