nvidiafb: access CRT registers safely

Use Read/WriteCrtc() to access CRTC registers in nv_i2c.c.  These are safer
  because it uses the correct CRTC base (0x3bx or 0x3dx).

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>