drm: switch drm_fb_xrgb8888_to_rgb888_dstclip to accept __iomem dst

Not all archs have the __io_virt() macro, so cirrus can't simply convert
pointers that way.  The drm format helpers have to use memcpy_toio()
instead.

This patch makes drm_fb_xrgb8888_to_rgb888_dstclip() accept a __iomem
dst pointer and use memcpy_toio() instead of memcpy().  The helper
function (drm_fb_xrgb8888_to_rgb888_line) has been changed to process a
single scanline.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190410063815.17062-4-kraxel@redhat.com