disp: msm: sde: check power event before set clk rate

Clock rate can be set from debugfs, and this can attempt to
set clock rate even when display power is not enabled.
Set clock rate should check the last power event first.

Change-Id: Ibf01753a288e5a3003928664c99aa6dbf26350d6
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>