nvidiafb: bring back generic ddc reading

Make nvidiafb use fb_ddc_read().  This patch was submitted before but was
reverted due to problems in a non-x86 platform.  This includes a fix for that
where ddc reading is bypassed if there is no DDC bus (duh).

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>