drm/i915/psr: Add debugfs support to force a downgrade to PSR1 mode.

This will make it easier to test PSR1 on PSR2 capable eDP machines.

Changes since v1:
- Remove I915_PSR_DEBUG_FORCE_PSR2, it did nothing, not sure forcing
  PSR2 would even work.
- Handle NULL crtc in intel_psr_set_debugfs_mode. (dhnkrn)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180808141911.7647-2-maarten.lankhorst@linux.intel.com


Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>