msm: mdss: fix debugfs offset setup

Offset read is providing incorrect offset count setup. The offset write
should take in hex value for count and have a default value in case none
given.

Change-Id: I8b11da07d84814d21727f4c997b485866531534f
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>