drm/msm: Add devfreq support

Try to initialize and use the 'simple_ondemand' devfreq governor.
If all the stars align the governor will poll every 10ms to
determine how busy the GPU is and adjust the frequency accordingly.

Change-Id: Ic0dedbad16d0733cbfd5b268fc14fa064192d864
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>