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: 992af0a4b8
       ("meson: dedup gallium-vdpau logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

meson.build

@@ -398,16 +398,17 @@ elif not (with_gallium_r300 or with_gallium_r600 or with_gallium_radeonsi or
   else
     _vdpau = 'false'
   endif
-elif _vdpau == 'auto'
-  _vdpau = 'true'
 endif
-with_gallium_vdpau = _vdpau == 'true'
 dep_vdpau = null_dep
-if with_gallium_vdpau
-  dep_vdpau = dependency('vdpau', version : '>= 1.1')
-  dep_vdpau = declare_dependency(
-    compile_args : run_command(prog_pkgconfig, ['vdpau', '--cflags']).stdout().split()
-  )
+with_gallium_vdpau = false
+if _vdpau != 'false'
+  dep_vdpau = dependency('vdpau', version : '>= 1.1', required : _vdpau == 'true')
+  if dep_vdpau.found()
+    dep_vdpau = declare_dependency(
+      compile_args : run_command(prog_pkgconfig, ['vdpau', '--cflags']).stdout().split()
+    )
+    with_gallium_vdpau = true
+  endif
 endif
 
 if with_gallium_vdpau