drm/i915/psr: pass i915 to psr_global_enabled()

Make future work slightly easier.

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204135217.21974-1-jani.nikula@intel.com