drm/i915: Document and future-proof preemption control policy

Intel hardware allows some preemption settings to be controlled either
by the kernel-mode driver exclusively, or placed under control of the
user-mode drivers; on Linux we always select the userspace control
option.  The various registers involved in this are not documented very
clearly; let's add some clarifying comments to help explain how this all
works and provide some history on why our Linux drivers take the
approach they do (which I believe differs from the path taken by certain
other operating systems' drivers).

While we're at it, let's also remove the graphics version 12 upper bound
on this programming.  As described, we don't have any plans to move away
from UMD control of preemption settings on future platforms, and there's
currently no reason to believe that the hardware will fundamentally
change how these registers and settings work after version 12.

Bspec: 45921, 45858, 45863
Cc: Joonas Lahtinen <[email protected]>
Cc: Jordan Justen <[email protected]>
Cc: Lionel Landwerlin <[email protected]>
Suggested-by: Joonas Lahtinen <[email protected]>
Signed-off-by: Matt Roper <[email protected]>
Reviewed-by: Wayne Boyer <[email protected]>
Acked-by: Tapani Pälli <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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