adreno_tz: Correct tz_buf pointer type to correct pointer arithmetic

Current tz_buf pointer type is causing the end address calcultion
of tz_buf to go wrong. "end_addr = tz_buf + PAGE_ALIGN(size)" is
resulting in an end address way beyond the allocated range because
tz_buf is of type unsigned int. This change changes the tz_buf
pointer type to u8 to fix this issue.

Change-Id: I16db09c565801b56c0c0ee8a8184f6e276512fa3
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>