imacfb: remove reference to otherwise-unused, non-existent screen_info.imacpm_seg

Cc: Edgar Hucek <hostmaster@ed-soft.at>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>