drm/mgag200: Embed connector instance in struct mga_device

Storing the connector instance in struct mga_device avoids some
dynamic memory allocation. On errors, the connector's initializer
function now destroys the i2c structure. Done in preparation of
converting mgag200 to simple-KMS helpers.

v2:
	* improved commit message (Michael)
	* fixed error message for mgag200_vga_connector_init() (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090315.21274-7-tzimmermann@suse.de