It's harmful to write to enable_psr at runtime, and the patch that allows
us to change i915_edp_psr_debug with the panel running will require us
to abandon the module parameter. Hence the userspace change needs to be
put in IGT first before we can change it at kernel time.

Toggling it to debugfs will mean we can skip a modeset when changing our
feature set.

Changes since v1:
- Rebase with the previous patches dropped.
Changes since v2:
- Rebase on top of new api in i915_edp_psr_debug.
Changes since v3:
- Enable IRQ debugging for extra logging.
- Force PSR1 mode. (dhnkrn)
- Move PSR enable/disable functions to lib/igt_psr. (dhnkrn)
Changes since v4:
- Redisable irqs right away when debugfs api doesn't work. (dhnkrn)
- Use hex everywhere. (dhnkrn)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Fix -ENODEV explanation in has_psr_debugfs (dhnkrn)]
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>