fbdev: *bfin*: drop unnecessary calls to memset

The dma_alloc_* functions sets the memory to 0 before returning so there
is no need to call memset after the allocation.  Also no point in clearing
the memory when disabling the buffer.

Signed-off-by: Vivek Kutal <vivek.kutal@azingo.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>