video: ssd1307fb: Use screen_buffer instead of screen_base

sparse reported incorrect type due to different address spaces.
The screen_base is __iomem, but the memory is not from a device so we can
use screen_buffer instead and avoid some type casts.

Signed-off-by: Marko Kohtala <marko.kohtala@okoko.fi>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618074111.9309-2-marko.kohtala@okoko.fi