drivers/video/Kconfig: Fix FB_PMAGB_B dependencies

Add a missing FB dependency to FB_PMAGB_B.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>