drm/vc4: Enable bg_fill if there are no planes enabled

The default was to have enable_bg_fill disabled and the first
plane set it if it wasn't opaque and covering the whole screen.
However that meant that if no planes were enabled, then the
background fill wasn't enabled, and would give a striped
output from the uninitialised output buffer.

Initialise it to enabled to avoid this.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>