drm/i915/params: add i915 parameters to debugfs

Add a debugfs subdirectory i915_params with all the i915 module
parameters. This is a first step, with lots of boilerplate, and not much
benefit yet.

This will result in a new device specific debugfs directory at
/sys/kernel/debug/dri/<N>/i915_params duplicating the module specific
sysfs directory at /sys/module/i915/parameters/. Going forward, all
users of the parameters should use the debugfs, with the module
parameters being phased out.

Add debugfs permissions to I915_PARAMS_FOR_EACH(). This duplicates the
mode with module parameter sysfs, but the goal is to make the module
parameters read-only initial values for device specific parameters.

0 mode will bypass debugfs creation. Use it for verbose_state_checks
which will need special attention in follow-up work.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/600101c8433e7caf9303663fc85a9972fa1f05e7.1575560168.git.jani.nikula@intel.com