drm/amdgpu: fix build for CONFIG_DRM_FBDEV_EMULATION=n

drm/amdgpu: fix build for CONFIG_DRM_FBDEV_EMULATION=n

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")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Yury Norov <ynorov@nvidia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>