drm/msm/phy/dsi_phy: Set pll to NULL in case initialization fails

We have if (!phy->pll) checks scattered through the driver and if
phy->pll is an error pointer, those checks will pass and bad things will
happen :(

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617200920.133104-1-sean@poorly.run