drm: Allow early kms with monitor EDID override.

drm: Allow early kms with monitor EDID override.

While firmware_put(fw, FIRMWARE_UNLOAD) works great when
fw can be loaded multiple times through kernel fw module,
this functionality breaks when loading i915 and radeonkms
together with EDID override module from /boot/loader.conf

Since DRM reads EDID firmware multiple times, it is best
to delay FW_UNLOAD to at least when / is accessable.
Doing that allows to have an early kms with a monitor
reporting bad EDID information at a native resolution.

EDID fw is usually just 128 bytes and still can be unloaded
if it was autoloaded by loader.

Tested-on: early i915 with eDP and broken external LCD panels.