[PATCH v2] drm/amdgpu: fix build for CONFIG_DRM_FBDEV_EMULATION=n

[Yury Norov <ynorov@nvidia.com>]

The merge-commit 02e778f12359 ("Merge tag 'amd-drm-next-7.1-2026-03-12' of
https://gitlab.freedesktop.org/agd5f/linux into drm-next") removes the stub
for drm_fb_helper_gem_is_fb(), so the buld gets broken if DRM_FBDEV_EMULATION
is not set.

‘drm_fb_helper_gem_is_fb’; did you mean ‘drm_fb_helper_from_client’? [-Wimplicit-function-declaration]
 1777 |                 if (!drm_fb_helper_gem_is_fb(dev->fb_helper, fb->obj[0])) {
      |                      ^~~~~~~~~~~~~~~~~~~~~~~
      |                      drm_fb_helper_from_client

Restore it.

Fixes: 02e778f12359 ("Merge tag 'amd-drm-next-7.1-2026-03-12' of https://gitlab.freedesktop.org/agd5f/linux into drm-next")
Signed-off-by: Yury Norov <ynorov@nvidia.com>
---
v2: restore only drm_fb_helper_gem_is_fb() (Thomas)

 include/drm/drm_fb_helper.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index bf391903443d..0c5e5ed7b5e7 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -273,6 +273,12 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper);
 int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper);
 bool drm_fb_helper_gem_is_fb(const struct drm_fb_helper *fb_helper,
 			     const struct drm_gem_object *obj);
+#else
+static inline bool drm_fb_helper_gem_is_fb(const struct drm_fb_helper *fb_helper,
+					   const struct drm_gem_object *obj)
+{
+	return false;
+}
 #endif
 
 #endif
-- 
2.51.0