meson: simplify dri3 logic

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -353,15 +353,9 @@ endif
 with_dri2 = (with_dri or with_any_vk) and with_dri_platform == 'drm'
 with_dri3 = get_option('dri3')
 if with_dri3 == 'auto'
-  if system_has_kms_drm and with_dri2
-    with_dri3 = true
-  else
-    with_dri3 = false
- endif
-elif with_dri3 == 'true'
-  with_dri3 = true
+  with_dri3 = system_has_kms_drm and with_dri2
 else
-  with_dri3 = false
+  with_dri3 = with_dri3 == 'true'
 endif
 
 if with_any_vk and (with_platform_x11 and not with_dri3)