drm/i915/psr: Move intel_psr_disable_source() code to intel_psr_disable_locked()

In the past we had hooks to configure HW for VLV/CHV too, in the drop
of VLV/CHV support the intel_psr_disable_source() code was not moved
to the caller, so doing it here.

Suggested-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106190843.18009-4-jose.souza@intel.com