drm/ssd130x: Fix screen clearing

Due to the reuse of buffers, ssd130x_clear_screen() no longers clears
the screen, but merely redraws the last image that is residing in the
intermediate buffer.

As there is no point in clearing the intermediate buffer and transposing
an all-black image, fix this by just clearing the HW format buffer, and
writing it to the panel.

Fixes: 49d7d581 ("drm/ssd130x: Don't allocate buffers on each plane update")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c19cd5a57205597bb38a446c3871092993498f01.1692888745.git.geert@linux-m68k.org