disp: pll: limit clock rate of shadow VCO clock

Limit clock rate of shadow VCO clock as normal VCO clock.
For larger bit clock rate gap between switched ones, the clock switching
would fail due to mismatched VCO clock rate between normal VCO clock and
shadow one.

Change-Id: I9d68725de360ac28c243a3ce1800bfb139f39757
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>