fbdev: remove unnecessary memset in vfb

In vfb_probe memory is allocated using rvmalloc which automatically
sets the allocated memory to zero. This patch removes the second
unnecessary memset in vfb_probe.

Signed-off-by: Marcin Chojnacki <marcinch7@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>