Use ->screen_buffer instead of ->screen_base in mmpfb driver.

[ Please see commit 17a7b0b4 ("fb.h: Provide alternate screen_base
  pointer") for details. ]

Also fix all other sparse warnings about using incorrect types in
mmp display subsystem.
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ee796b43-f200-d41a-b18c-ae3d6bcaaa67@samsung.com