drm/radeon: set default bl level to something reasonable

If the value in the scratch register is 0, set it to the
max level.  This fixes an issue where the console fb blanking
code calls back into the backlight driver on unblank and then
sets the backlight level to 0 after the driver has already
set the mode and enabled the backlight.

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=81382
https://bugs.freedesktop.org/show_bug.cgi?id=70207



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: David Heidelberger <david.heidelberger@ixit.cz>
Cc: stable@vger.kernel.org