drm/msm/dsi-staging: Add range check in debugfs_dump_info_read

Add a check to make sure that the length of bytes copied
to the destination buffer doesn't exceed the requested
buffer length before calling the copy_to_user to avoid
buffer overflow.

Change-Id: Icd65b9be2791a8a487dfc8d7461aadce61de3f1b
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>