OMAPDSS: skip pclk check for WB mem2mem

OMAPDSS: skip pclk check for WB mem2mem

When doing mem-to-mem writeback, there's no pixelclock. However, the
code that calculates scaling factors check that there is a pixel clock.
We can just skip the check when doing mem-to-mem writeback.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>