fbdev/ssd1307fb: clear screen in probe

SSD1306 does not clear the panel's framebuffer automatically, even if a
HW reset happens, so we need to do that at probe time before enabling
the panel.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Benoît Cousson <bcousson@baylibre.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>