meson: avoid changing types for the dri3 option

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/meson.build b/meson.build
index 40e9c8f..308f64c 100644
--- a/meson.build
+++ b/meson.build
@@ -371,11 +371,11 @@
 endif
 
 with_dri2 = (with_dri or with_any_vk) and with_dri_platform == 'drm'
-with_dri3 = get_option('dri3')
-if with_dri3 == 'auto'
+_dri3 = get_option('dri3')
+if _dri3 == 'auto'
   with_dri3 = system_has_kms_drm and with_dri2
 else
-  with_dri3 = with_dri3 == 'true'
+  with_dri3 = _dri3 == 'true'
 endif
 
 if with_any_vk and (with_platform_x11 and not with_dri3)