video/fb: cleanup FB_MAJOR usage

Currently, linux/major.h defines a GRAPHDEV_MAJOR (29) that nobody uses,
and linux/fb.h defines the real FB_MAJOR (also 29), that only fbmem.c
needs.  Drop GRAPHDEV_MAJOR from major.h, move FB_MAJOR definition from
fb.h to major.h, and fix fbmem.c to use major.h's definition.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
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>