fsl_diu: fix alignment error that caused malloc corruption

When aligning malloc()ed screen_base, invalid offset was added.
This not only caused misaligned result (which did not cause hardware
misbehaviour), but - worse - caused screen_base + smem_len to
be out of malloc()ed space, which in turn caused breakage of
futher malloc()/free() operation.

This patch fixes screen_base alignment.

Also this patch makes memset() that cleans framebuffer to be executed
on first initialization of diu, not only on re-initialization. It looks
correct to clean the framebuffer instead of displaying random garbage;
I believe that was disabled only because that memset caused breakage
of malloc/free described above - which no longer happens with the fix
described above.

Signed-off-by: Nikita V. Youshchenko <yoush@debian.org>