Allow up to six instances in Zaphod mode

Corresponding to up to six CRTCs being available in the hardware.

v2:
* Move instance overflow check from PreInit to the probe hooks, in
  order to further minimize wasted effort.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com> # v1
11 jobs for zaphod-more-instances in 2 minutes and 2 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #28547
xserver-1.13

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xserver-1.14

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passed #28549
xserver-1.15

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xserver-1.16

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xserver-1.17

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passed #28552
xserver-1.18

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passed #28553
xserver-1.18-clang

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xserver-1.19

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xserver-1.19-clang

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passed #28545
xserver-1.20

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passed #28546
xserver-1.20-clang

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