drm/vc4: clean up error exit path on failed dpi_connector allocation

There is redundant code in the clean up exit path when dpi_connector
fails to be allocated.  The current code checks if connector is NULL
before destroying it, in fact, connector is NULL at this point so
the check is redundant and can be removed. The final clean up is
that we can remove the goto fail with a simple return and the unused
variable ret.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>