gpu/intel: Precisely compute the period exponent

Use the formula as specified in the intel drm header.

Signed-off-by: Corentin Noël <corentin.noel@collabora.com>
Reviewed-by: Antonio Caggiano <antonio.caggiano@collabora.com>
3 jobs for tintou/oa_exponent in 2 minutes and 6 seconds (queued for 1 second)
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passed #7907185
clang-format

00:00:09

 
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aarch64
aarch64

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passed #7907186
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x86_64

00:01:15