drm/i915/psr: Timestamps for PSR entry and exit interrupts.

Timestamps are useful for IGT tests that trigger PSR exit and/or wait for
PSR entry.

v2: Removed seqlock (Ville)
    Removed erroneous warning in irq loop (Chris)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jose Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180403212420.25007-4-dhinakaran.pandiyan@intel.com