drm/omap: sdi: Fixup video mode in .check_timings() operation

The SDI encoder modifies the pixel clock of the requested video mode to
take the limitations of the PLL into account in the .enable() operation.
This should be performed in the .check_timings() operation instead. Move
the fixup.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>