fbdev: fbmem: allow overriding the number of bootup logos

Probably most useful if you want no logo at all, or if you only want one
logo regardless of how many CPU cores you have.

Signed-off-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827110854.12574-3-peda@axentia.se