fbdev: simplefb: fix Kconfig dependencies

Configurations with both CONFIG_FB_SIMPLE=y and CONFIG_DRM_SIMPLEDRM=m
are allowed by Kconfig because the 'depends on !DRM_SIMPLEDRM' dependency
does not disallow FB_SIMPLE as long as SIMPLEDRM is not built-in. This
can however result in a build failure when cfb_fillrect() etc are then
also in loadable modules:

x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x1f8): undefined reference to `cfb_fillrect'
x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x200): undefined reference to `cfb_copyarea'
x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x208): undefined reference to `cfb_imageblit'

To work around this, change FB_SIMPLE to be a 'tristate' symbol,
which still allows both to be =m together, but not one of them to
be =y if the other one is =m. If a distro kernel picks this
configuration, it can be determined by local policy which of
the two modules gets loaded. The 'of_chosen' export is needed
as this is the first loadable module referencing it.

Alternatively, the Kconfig dependency could be changed to
'depends on DRM_SIMPLEDRM=n', which would forbid the configuration
with both drivers.

Fixes: 11e8f5fd223b ("drm: Add simpledrm driver")
Acked-by: Rob Herring <[email protected]> # for drivers/of/
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Daniel Vetter <[email protected]> # fbdev support
Cc: Maxime Ripard <[email protected]>
Cc: Liam Girdwood <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Javier Martinez Canillas <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: <[email protected]> # v5.14+
Signed-off-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Maarten Lankhorst <[email protected]>

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