MLK-16581-2 phy: mixel-lvds-combo: Get PHY clock rate before setting it's rate

MLK-16581-2 phy: mixel-lvds-combo: Get PHY clock rate before setting it's rate

Due to i.MX8 clock issue, we need to get PHY clock rate
before setting it's rate when system resumes back from
PM sleep mode, otherwise, we'll fail to set the clock rate.
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>
(cherry picked from commit fca3d4b1074fbc3ca74db040f6abdf9b614df819)