drm/r128/ati_pcigart: Source file headers are not good candidates for kernel-doc

drm/r128/ati_pcigart: Source file headers are not good candidates for kernel-doc

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

 drivers/gpu/drm/r128/ati_pcigart.c:2: warning: Cannot understand  * file ati_pcigart.c

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Gareth Hughes <gareth@valinux.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106214949.2042120-3-lee.jones@linaro.org