[REGRESSION][BISECTED] 5.9-rc4 disables console on radeon

[Mikael Pettersson]

Starting with linux-5.9-rc4, the Dell monitor on my desktop PC goes
black during boot
when the kernel activates the framebuffer console, except for this
error message shown
in the center of the screen:

"Dell U2412M
 The current input timing is not supported by the monitor display. Please
 change your input timing to 1920x1200 at 60Hz or any other monitor
 listed timing as per the monitor specifications.
 <DVI symbol>"

The monitor remains black until I reboot.

All kernels up to and including 5.9-rc3 were Ok.  A git bisect identified

# first bad commit: [fc8c70526bd30733ea8667adb8b8ffebea30a8ed]
drm/radeon: Prefer lower feedback dividers

as the culprit, and reverting that from -rc4 makes the console work again.

Adding a bit of debugging code to that function shows:

avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137
avivo_get_fb_ref_div: fb_div_new 142 fb_div_old 143
avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137
avivo_get_fb_ref_div: fb_div_new 119 fb_div_old 120
avivo_get_fb_ref_div: fb_div_new 136 fb_div_old 137

during boot, where "new" is what the commit above changed the code to compute,
and "old" is the value computed by the working code from rc3.

The graphics card is a Radeon HD6450 silent model:
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
[AMD/ATI] Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM] (prog-if 00
[VGA controller])