viafb: Reserve framebuffer memory for the upcoming camera driver

The camera engine captures to framebuffer memory, so we need to set some
aside for that purpose.  There is no proper memory allocator for fbmem;
instead, accel.c just trims some space off the top.  Alas, without creating
that proper memory allocator, the only way to make this work is to hack it
into the same bit of code in accel.c.  The allocation must happen *after*
the others (some code, including user-space XV stuff, makes assumptions on
where the cursor space is), and before the rest of the framebuffer is set
up.

Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Cc: Harald Welte <laforge@gnumonks.org>
Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>