drm/amdgpu: remove redundant null check

Remove the redundant null check since the caller ensures
that 'ctx' is never NULL.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Weiguo Li <liwg06@foxmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>