drm/amdgpu: add missing df counter disable write

Request to stop DF performance counters is missing the actual write to the
controller register.

Reported-by: Chris Freehill <chris.freehill@amd.com>
Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harish Kasiviswanathan <harish.kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>