drm/i915/psr: Use intel_psr_exit() in intel_psr_disable_source()

Both functions have the same code to disable PSR, so let's reuse that
code instead of duplicate.

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/20181106190843.18009-1-jose.souza@intel.com