meson: don't require glx/egl/gbm with gallium drivers

The gallium drivers do not require a DRI loader. Drop the artificial
and unnecessary restriction.

Fixes: af9d276134d ("meson: build libmesa_gallium")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/meson.build b/meson.build
index 5515f26..9ecb8ea 100644
--- a/meson.build
+++ b/meson.build
@@ -381,9 +381,9 @@
 if with_any_vk and (with_platform_x11 and not with_dri3)
   error('Vulkan drivers require dri3 for X11 support')
 endif
-if with_dri or with_gallium
-  if with_glx == 'disabled' and not with_egl and not with_platform_haiku
-    error('building dri or gallium drivers require at least one window system')
+if with_dri
+  if with_glx == 'disabled' and not with_egl and not with_gbm
+    error('building dri drivers require at least one windowing system')
   endif
 endif