phy: dphy: Remove unused header

The videomode.h header inclusion is an artifact from the patches
development, remove it.

Suggested-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>