drm: omapdrm: gem: Move global usergart variable to omap_drm_private

The structure contains data related to a device instance, it shouldn't
be a global variable.

While at it rename the usergart structures with an omap_drm_ prefix.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>