MAINTAINERS: Update tree location for the Renesas DRM drivers

The fbdev git tree referenced in the MAINTAINERS file doesn't exist
anymore. Update the location to point to the new git tree.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>