intelfb: update doc and Kconfig (supported devices)

According to drivers/video/intelfb/intelfb.h, the intelfb driver
supportes the following devices:
830M/845G/852GM/855GM/865G/915G/915GM/945G/945GM. So the description in
drivers/video/Kconfig and the documentation in
Documentation/fb/intelfb.txt is outdated.

airlied: cleaned up some other obvious mistakes in intelfb.txt.

Signed-off-by: Christian Merkle <mail@christian-merkle.de>
Signed-off-by: Dave Airlie <airlied@linux.ie>