techpack: disp: pll: power on PLL when reading PLL registers

Reading register of DSI PLL would fail if PLL is not powered
up, so enable DSI PLL power before reading operation.

Change-Id: Ie9933745035392744bb49241342f05a6be9daedc
Signed-off-by: Ray Zhang <rayz@codeaurora.org>
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>