drm/i915/pxp: add pxp debugfs

2 debugfs files, one to query the current status of the pxp session and one
to trigger an invalidation for testing.

v2: rename debugfs, fix date (Alan)

v12: rebased to latest drm-tip (rename of files/structs from
     debugfs_gt to intel_debugfs_gt caused compiler errors).

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by : Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924191452.1539378-16-alan.previn.teres.alexis@intel.com