OMAPDSS: convert pixel clock to common videomode style

omapdss has its own video-timings struct, but we want to move the common
videomode.

The first step is to change the omapdss's pixelclock unit from kHz to
Hz. Also, omapdss uses "pixel_clock" field name, whereas the common
videomode uses "pixelclock" field name. This patch changes the field
name also, as that makes it easy to spot any non-converted pixel_clock
uses.

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