msm: mdss: increase xlog entries for debug purpose

Print 256 xlog entries in kernel log but keep 1024
entries in memory. This will be helpful while debugging
issue where user needs larger xlog information.

Change-Id: Ic8b6143eef7afd1220ccf40f8ad7617b94017532
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>