fbdev/amifb: Use framebuffer_alloc()

Use framebuffer_alloc() instead of static fb_info and currentpar variables.
Also sanitize the error path and cleanup code (e.g. missing free_irq()).
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>