drm/i915/gem: fix function pointer member kernel-doc

You can't document function pointer member as functions.

drivers/gpu/drm/i915/gem/i915_gem_region.h:25: warning: Incorrect use of kernel-doc format:          * process_obj - Process the current object
drivers/gpu/drm/i915/gem/i915_gem_region.h:35: warning: Function parameter or member 'process_obj' not described in 'i915_gem_apply_to_region_ops'

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a2a81f9ee78591def0534c81a63dbc652c44bbd3.1683041799.git.jani.nikula@intel.com