drm/msm/dsi: Save/Restore PLL status across PHY reset

Reset DSI PHY silently changes its PLL registers to reset status,
which will make cached status in clock driver invalid and result
in wrong output rate of link clocks. The current restore mechanism
in DSI PLL does not cover all the cases. This change is to recover
PLL status after PHY reset to match HW status with cached status
in clock driver.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>