dma-buf: use hashtable to store dmabufs in dmaprocs

dmaprocs uses list to store all dmabufs of a process, during iterate_fd
we loop through all the files of a process and check if each file is a
dmabuf and if it is dmabuf we check if the list already contains this
dmabuf we do this while holding a file_lock this could take time if the
list is large and cause contention on the file_lock, so replace list
with a hashtable so that this time is reduced.

Change-Id: Ib65863530161ecef1bf67cc85fbe3de0b6e3d342
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>