drm/omap: Use consistent name for struct videomode

Use 'vm' to refer to a struct videomode instead of 'p', 't', 'timings' or
something else.

The code will be easier to follow if we use consistent names.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>