msm: mdss: Check for buffer boundary condition in panel_debug_reg_write

- Before reading from new offset in a buffer, check for out
of bounds condition.
- Avoid using '%n' specifier.
- Use kstrtouint() instead of single variable sscanf
as per coding guidelines.

Change-Id: I10ea6f2b22d554d02f302f5700f6674d08e4777d
Signed-off-by: Krishna Srinivas <krisrini@codeaurora.org>