x11-drivers/nvidia-drivers: check for CONFIG_DRM_FBDEV_EMULATIONHEADmaster

Should've known about this given the TTM bit, but kind of forgot.

Keep it optional, like the rest of the DRM bits a system can work
without these if you know what you're doing (e.g. disabled KMS_HELPER
needs modeset=0 and a custom Xorg.conf). TTM on the other hand is
required *if* FBDEV is enabled else it results in a build failure
on recent kernels.

Plans is mostly to make most people enable it before we do the switch
"eventually".

For what it's worth, only check in 570+ given there's no plans to ever
enable fbdev=1 by default in old branches, legacy ones did not even
support it.

Bug: https://bugs.gentoo.org/949097
Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

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