drm/i915/psr: Initialize PSR mutex even when sink is not reliable

Even when driver is reloaded and hits this scenario the PSR mutex
should be initialized, otherwise reading PSR debugfs status will
execute mutex_lock() over a mutex that was not initialized.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190406005112.27205-3-jose.souza@intel.com