drm/amdgpu: fix adev variable used in amdgpu_device_gpu_recover()

Use the correct adev variable for the drm_fb_helper in
amdgpu_device_gpu_recover().  Noticed by inspection.

Fixes: 087451f3

 ("drm/amdgpu: use generic fb helpers instead of setting up AMD own's.")
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org