msm: mdss: add pll master/slave auto detection for msm8996

Set up pll master/slave status based on both
GLBL_TEST_CTRL and CLKBUFLR_EN pll registers value.

Change-Id: Ic75e77d5fccb88613ebe435f97a39f5d745fb264
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>