msm: mdss: debugfs: xlog: fix xlog corruption in ramdump

Display driver debug feature xlog, allocates some memory
to copy some of the MDP registers that are needed for
debugging and then it panics. Current memory allocation
api has a problem, where data from this memory allocation
is shown corrupted in the ramdump.
This change workaround this problem by changing the
memory allocation to a dma alloc which works fine
to keep the data correct in the ramdump.

Change-Id: I391419095fb7ebfd3eb88faf26d2b473c255f28c
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>