imx8mp-ldb: remove hardcoded pixel clock overwrite

The imx8mp ldb driver had an hardcoded overwrite for the pixelclock as
the video_pll is not able to generate all needed frequencies.
But the hardcoded value only fits to one mode, probably used on the
EVK.
Removing this override leads to a pixelclock that may be generated with
a devider from the PLL, which ends up with a frquency much closer to what
is expected.

Also it is not the ldb clock but the lcdif clock used for this, so this
seems to be the wrong place for modifying the clock anyway.

BCS 746-000502

(cherry picked from commit 86df943d)