OMAPDSS: HDMI: Correct source of the pixel clock in ACR calculation

Due to changes in struct hdmi_config, the pixel clock has to be obtained
differently.

The pixel clock is needed to calculate the CTS value as defined in the
HDMI specification.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>