drm/msm: add sde power handler as a client of rsc

Make sde power handler as a client of rsc, as this would
also influence rsc's mode2 entry/exit apart from the encoders.
Request for RSC with CLK_MODE request during power handler
enable and IDLE_MODE during disable.

Change-Id: I61c5509be941cc26946d1a9eb5aef4691ecdb00a
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>