drm/msm/dsi: do not try reading 28nm vco rate if it's not enabled

Reading VCO rate for this PLL can cause boot stalls, if it is not
enabled. Guard clk_hw_get_rate with a call to
dsi_pll_28nm_clk_is_enabled().

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>