drm/i915/gem: placate scripts/kernel-doc

Correct kernel-doc warnings in i915_drm_object.c:

i915_gem_object.c:103: warning: expecting prototype for i915_gem_object_fini(). Prototype was for __i915_gem_object_fini() instead
i915_gem_object.c:110: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Mark up the object's coherency levels for a given cache_level
i915_gem_object.c:110: warning: missing initial short description on line:
 * Mark up the object's coherency levels for a given cache_level
i915_gem_object.c:457: warning: No description found for return value of 'i915_gem_object_read_from_page'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211123050928.20434-1-rdunlap@infradead.org