meson: Correct behavior of vdpau=auto

Currently if vdpau is set to auto, it will be disabled only in cases
where gallium is disabled or the host OS is not supported (mac, haiku,
windows). However on (for example) Linux if libvdpau is not installed
then the build will error because of the unmet dependency. This corrects
auto to do the right thing, and not error if libvdpau is not installed.

Fixes: 992af0a4
       ("meson: dedup gallium-vdpau logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit d9a8008a)