meson: Fix auto option for xvmc

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

Fixes: 724916c8a84b5bba8f880f17da936585d52c97b6
       ("meson: dedup gallium-xvmc logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/meson.build b/meson.build
index 429865c..82e2b53 100644
--- a/meson.build
+++ b/meson.build
@@ -438,13 +438,12 @@
   else
     _xvmc = 'false'
   endif
-elif _xvmc == 'auto'
-  _xvmc = 'true'
 endif
-with_gallium_xvmc = _xvmc == 'true'
 dep_xvmc = null_dep
-if with_gallium_xvmc
-  dep_xvmc = dependency('xvmc', version : '>= 1.0.6')
+with_gallium_xvmc = false
+if _xmvc != 'false'
+  dep_xvmc = dependency('xvmc', version : '>= 1.0.6', required : _xvmc == 'true')
+  with_gallium_xvmc = dep_xvmc.found()
 endif
 
 xvmc_drivers_path = get_option('xvmc-libs-path')