msm: mdss: recalc pclk for Y420 format

Pixel clock should be 1/2 for Y420 format display.
Add check for Y420 format support and do pclk
calculation accordingly.

CRs-Fixed: 2071321
Change-Id: I49b366dbbf1f36f147afed3b238363f94e612b37
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>