drivers/video/fbmem.c: simplify strlen()==0 check in fb_get_options()

Replaced !strlen(str) check with !str[0].  Removed the variable which was
used solely to store strlen result.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>