drm/vc4: clean up error handling on devm_kzalloc failure

The current error handling when devm_kzalloc fails performs a
non-null check on connector which is redundant because connector
is null at that failure point.  Once this is removed, make the
failure path into a trivial -ENOMEM return to clean up the
error handling. Also remove need to initialize connector to NULL.

Detected by CoverityScan CID#1339527 ("Logically dead code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908140504.1340-1-colin.king@canonical.com