drm/vc4: simplify exit path of a failed allocation of dsi_connector

If dsi_connector fails to allocate, the exit path via label 'fail'
checks if connector is null, which it always is, so the cleanup
that destroys connector is never going to be called.  Hence the
failure path can be more optimally performed by removing this
and just returning ERR_PTR(-ENOMEM).  This also removes the need
to initialize connector to NULL, and we can also remove ret too.

Detected by CoverityScan, CID#1399504 ("Logicall Dead Code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170203195624.7189-1-colin.king@canonical.com