msm: dcvs: remove idle notification registration

Register the idle enable callback along with the core. The code
becomes cleaner and easy to update.

Importantly, the msm_dcvs_idle driver becomes useless. Remove it
and instead let the msm governor handle idle enabling and disabling.

Change-Id: Ia056b8b1fcca8ebd356fa0484148c3ade54026fc
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>