drm/omap: use DRM_DEBUG_DRIVER instead of CORE

This macro is only used by omapdrm, which should print
debug messages using the DRIVER category instead of the
default CORE category.

Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>