ENGR00288021 IPUv3 disp:get the integer part of div correctly

ENGR00288021 IPUv3 disp:get the integer part of div correctly

TV will show 639x480p60 when HDMI output 640x480p60.
The same issue can be found when pixel clock sources from ipu internally.
All video modes whose pixel clocks derive from DI clock work OK.
It is caused by the wrong parent clock rate the driver gets.
Fix the issue by getting the right parent clock(ipu->pixel_clk_sel).

Signed-off-by: Sandor Yu <R01008@freescale.com>