drm/msm: Register GPU devfreq to allow thermal mitigation

To allow thermal framework to scale the GPU frequency to mitigate
the GPU temperature. Register GPU as a devfreq based cooling device.

Change-Id: I01176ef96a69f1aa90d30f14f5c9d940e97cc188
Signed-off-by: Felix Xiong <xayang@codeaurora.org>
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>