msm: thermal: Decouple Gfx phase control monitoring

Decouple Gfx phase control monitoring. With this
change, the temperature band transition from
hot-critical to warm and from warm to normal
temperature can be monitored independently.

CRs-fixed: 703511
Change-Id: I64a6130851f4c9641ec01d7ecfc8848ee5c7a9fa
Signed-off-by: Shiju Mathew <shijum@codeaurora.org>