drm: vc4: remove redundant check of plane being non-null

The pointer plane is always null on the error path at label 'fail'
hence the check if it is non-null is redundant. We can therefore
remove the check and the destruction of plane as well as the fail
error path and instead just return an -ENOMEM ERR_PTR.

Detected by CoverityScan, CID#1339532 ("Logically Dead Code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170316185418.32765-1-colin.king@canonical.com