fbdev: Hot-unplug firmware fb devices on forced removal

Hot-unplug all firmware-framebuffer devices as part of removing
them via remove_conflicting_framebuffers() et al. Releases all
memory regions to be acquired by native drivers.

Firmware, such as EFI, install a framebuffer while posting the
computer. After removing the firmware-framebuffer device from fbdev,
a native driver takes over the hardware and the firmware framebuffer
becomes invalid.

Firmware-framebuffer drivers, specifically simplefb, don't release
their device from Linux' device hierarchy. It still owns the firmware
framebuffer and blocks the native drivers from loading. This has been
observed in the vmwgfx driver. [1]

Initiating a device removal (i.e., hot unplug) as part of
remove_conflicting_framebuffers() removes the underlying device and
returns the memory range to the system.

[1] https://lore.kernel.org/dri-devel/[email protected]/

v2:
	* rename variable 'dev' to 'device' (Javier)

Signed-off-by: Thomas Zimmermann <[email protected]>
Reported-by: Zack Rusin <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Reviewed-by: Zack Rusin <[email protected]>
Reviewed-by: Hans de Goede <[email protected]>
CC: [email protected] # v5.11+
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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