MLK-16581-3 drm/imx: ldb: Get bypass & pixel clock rates before setting their rates

Due to i.MX8 clock issue, we need to get bypass and pixel clock rates
before setting their rates when system resumes back from PM sleep mode,
otherwise, we'll fail to set the clock rates.  So, this is a workaround
and it can be removed when the clock issue is properly fixed.

Signed-off-by: Liu Ying <victor.liu@nxp.com>