meson: simplify the gbm option code, and avoid changing types

v2: drop gallium comment (Dylan)

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 6f2615f..40e9c8f 100644
--- a/meson.build
+++ b/meson.build
@@ -294,16 +294,14 @@
   with_shared_glapi = false
 endif
 
-with_gbm = get_option('gbm')
-if with_gbm == 'auto' and with_dri  # TODO: or gallium
-  with_gbm = system_has_kms_drm
-elif with_gbm == 'true'
-  if not system_has_kms_drm
-    error('GBM only supports DRM/KMS platforms')
-  endif
-  with_gbm = true
+_gbm = get_option('gbm')
+if _gbm == 'auto'
+  with_gbm = system_has_kms_drm and with_dri
 else
-  with_gbm = false
+  with_gbm = _gbm == 'true'
+endif
+if with_gbm and not system_has_kms_drm
+  error('GBM only supports DRM/KMS platforms')
 endif
 
 _egl = get_option('egl')