meson: Only error building gallium video without libdrm when the platform is drm

Fixes: 3b265f61f5f61f08718fe5bb4b2726f9b8e016cc
       ("meson: gallium media state trackers require libdrm with x11")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1878
Tested-by: Vinson Lee <vlee@freedesktop.org>
diff --git a/meson.build b/meson.build
index 5ceda5c..bce17a7 100644
--- a/meson.build
+++ b/meson.build
@@ -1443,7 +1443,7 @@
         with_gallium_omx != 'disabled'))
     dep_xcb = dependency('xcb')
     dep_x11_xcb = dependency('x11-xcb')
-    if not dep_libdrm.found()
+    if with_dri_platform == 'drm' and not dep_libdrm.found()
       error('libdrm required for gallium video statetrackers when using x11')
     endif
   endif