drm/i915/psr: Add missing check for I915_PSR_DEBUG_IRQ bit

We print the last attempted entry and last exit timestamps only when
IRQ debug is requested. This check was missed when new debug flags were
added in 'commit c44301fc ("drm/i915: Allow control of PSR at
runtime through debugfs, v6")

Fixes: c44301fc ("drm/i915: Allow control of PSR at runtime through
debugfs, v6")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180821221156.2442-2-dhinakaran.pandiyan@intel.com