radeonfb: fix debug option

Fix CONFIG_FB_RADEON_DEBUG.  DEBUG must be defined before including any kernel
header, otherwise dev_dbg() resolves to a no-op.  Also, when debugging is
disabled, don't set DEBUG at all instead of setting it to 0, to comply with
what the kernel headers expect.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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>