drm/i915/gem/i915_gem_create: Provide the function names for proper kerneldoc headers

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/i915/gem/i915_gem_create.c:147: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/gpu/drm/i915/gem/i915_gem_create.c:218: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/gpu/drm/i915/gem/i915_gem_create.c:402: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

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: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[Jani: fixed i915_gem_create_ext_ioctl while applying]
Link: https://patchwork.freedesktop.org/patch/msgid/20230331092607.700644-8-lee@kernel.org