drm/i915/psr/skl+: Print information about what caused a PSR exit

This will be helpful to debug what hardware is actually tracking
and causing PSR to exit.

BSpec: 7721

v4:
- Using _MMIO_TRANS2() in PSR_EVENT
- Cleaning events before printing

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425212334.21109-2-jose.souza@intel.com