meson: Fix auto option for xvmc

This fixes the same problem as the previous patch did for vdpau, but for
xvmc.

Fixes: 724916c8
       ("meson: dedup gallium-xvmc logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit a6943bb4)

Squashed with:

meson: Fix typo that breaks -Dgalium-xvmc=false

_xmvc -> _xvmc. Sigh

Fixes: a6943bb4
       ("meson: Fix auto option for xvmc")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Clayton Craft <clayton.a.craft@intel.com>
(cherry picked from commit ced3df56)