fbdev: Fix loading of module radeonfb on PowerMac

fbdev: Fix loading of module radeonfb on PowerMac

When the linux kernel is build with (typical kernel ship with Debian
installer):

CONFIG_FB=y
CONFIG_FB_OF=y
CONFIG_VT_HW_CONSOLE_BINDING=y
CONFIG_FB_RADEON=m

The offb driver takes precedence over module radeonfb. It is then
impossible to load the module, error reported is:

[   96.551486] radeonfb 0000:00:10.0: enabling device (0006 -> 0007)
[   96.551526] radeonfb 0000:00:10.0: BAR 0: can't reserve [mem 0x98000000-0x9fffffff pref]
[   96.551531] radeonfb (0000:00:10.0): cannot request region 0.
[   96.551545] radeonfb: probe of 0000:00:10.0 failed with error -16

This patch reproduce the behavior of the module radeon, so as to make it
possible to load radeonfb when offb is first loaded, see
commit a56f7428d753 ("drm/radeon: Add early unregister of firmware fb's").

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Link: https://bugs.debian.org/826629#57
Link: https://bugzilla.kernel.org/show_bug.cgi?id=119741
Suggested-by: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>