drm/i915/psr: Share PSR and PSR2 exit mask

Now both PSR and PSR2 have the same exit mask, so let's share then
instead of have the same code 2 times.

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003205031.32474-1-jose.souza@intel.com