sm501fb: clear framebuffer memory and palette

sm501fb: clear framebuffer memory and palette

Avoid displaying garbage on unused framebuffers.  For most users a single
framebuffer is used together with fbcon.  sm501fb supports two framebuffers
where one often is assigned to fbcon and the other one is left unused during
the boot.

The problem here is that framebuffers not in use by fbcon happen to display
garbage.  This can easily be solved by making sure that framebuffer memory and
palette ram are cleared.

The problem can be observed by using looking at the panel output (fb1) after
booting the kernel with fbcon on crt (fb0).  This is the default
configuration.  It's also possible to watch the garbage on the crt framebuffer
by passing "fbcon=map:1" on the kernel cmdline.  This will assign fbcon to the
panel (fb1) and leave the crt (fb0) unused.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>