touchscreen: atmel_mxt_ts: support multiple instances of T9 object

When calculating the minimum report ID, the number of report IDs
for each instance need to be accounted for.

Change-Id: Ib75ce79c732a5b8591324a3a7bb5e83a4c37aba4
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>