drm: fix drm_fb_xrgb8888_to_rgb888_dstclip()

Oops, the __iomem annotation was added to the header file only.
Add it to the implementation (and documentation) too.

Fixes: 5c5373b5 ("drm: switch drm_fb_xrgb8888_to_rgb888_dstclip to accept __iomem dst")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190411044932.13247-1-kraxel@redhat.com