drm/i915/pxp: fix sparse warning for not declared symbol

Fix:

drivers/gpu/drm/i915/pxp/intel_pxp_debugfs.c:61:6: warning: symbol
'intel_pxp_debugfs_register' was not declared. Should it be static?

Sort and remove the redundant pxp prefixes from the includes while at
it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220506120405.2582372-1-jani.nikula@intel.com