arm: vp8: Fix the plain-armv6 version of vp8_luma_dc_wht

This makes the plain-armv6 version use the same registers as the
armv6t2 version above.

This fixes fate-vp8 on plain-armv6 devices.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 2026eb1408a718c37835eb4b258c63714ab3205e)

Signed-off-by: Martin Storsjö <martin@martin.st>