Lost hardware acceleration from >=drm-515-kmod for Cherryview and Valleyview i915kms

Hello everyone!

My name is Roger and me and other people from the FreeBSD Forums have 
noticed that the hardware acceleration for Cherryview and Valleyview 
fails from drm-515-kmod and upwards to drm-latest. This is for i915kms.

I have posted a PR in Bugzilla with symptoms - 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295788

On a 14.1-RELEASE with drm-510-kmod the hardware acceleration works.

On same 14.1-RELEASE with drm-515 the hardware acceleration does not work.

I am aware that 14.1 is an unsupported system but it's the last known 
RELEASE that is supported by drm-510-kmod. Also drm-510-kmod is about to 
vanish, it's clinging in ports 2026Q2.

Other RELEASES like 14.2 up to 15.0-p9 behaves exactly the same when 
using drm-515-kmod or any other drm-kmod. Software rendering works but 
there is no hardware acceleration.

The source code for all drm versions contain the code for Cherryview and 
Valleyview.

I have read that there was a major rewrite of the code for the atomic 
display framework and sysfs structures for the drm i915 driver when 
upgrading from 5.10 to 5.11 on the Linux side. Maybe this is where a bug 
has crept in that provides hardware acceleration?

I hope that some kind soul can find the time to look into this(?) - I 
can help with testing and give vague promises of free beers ;^D

/Roger