vc4/drm: Remove the clear of SCALER_DISPBKGND_FILL

Since "drm/vc4: hvs: Support BCM2712 HVS" booting Pi4
with dual 4kp30 displays connected fails with:
vc4-drm gpu: [drm] *ERROR* [CRTC:107:pixelvalve-4] flip_done timed out

It has been tracked down to the referenced commit adding a
path to clear the SCALER_DISPBKGND_FILL when not required.

Dual 4kp30 works with a core clock of 297MHz when background fill
is enabled, but requires a higher value with it disabled.
320MHz still fails, while 330MHz seems okay.

Lets always enable background fill for Pi0-4.

Fixes: e84da235

 ("drm/vc4: hvs: Support BCM2712 HVS")

Signed-off-by: Dom Cobley <popcornmix@gmail.com>