anv: restore the old behavior of up to 75% of RAM for the system heap

"We paid for sixteen gigs of RAM, so we gonna use the whole damn
   sixteen gigs of RAM!"
     - My Mom

First, some history:

The Anv 50%-or-75% rule was originally added in 2017 by 060a6434ec
("anv: Advertise larger heap sizes"). When i915.ko started reporting
memory sizes in its ioctls, it didn't impose any restrictions: 100% of
SRAM was reported as available, so the restriction was in Mesa. When
xe.ko was introduced, it only reported 50% of the SRAM as available
through its ioctls, so commit b571ae6e7a ("intel: Make memory heaps
consistent between KMDs") adapted the code to not take an extra 25% of
the 50% that was already cut, and restricted i915.ko to 50% instead of
the 50%-or-75%. In Kernel commit d2d5f6d57884 ("drm/xe: Increase the
XE_PL_TT watermark"), xe.ko changed to reporting 100% of SRAM through
its ioctls, so we adapted Mesa to do the right thing depending on
which Kernel version was running.

While this was all happening, we were discussing about which behavior
was actually the best: restrict everything to 50% in order to avoid
issues when many things are running in parallel, or keep the
restriction only at 75% in order to allow high demanding workloads to
make full use of the hardware.

The way I see, if parallel applications are causing the system to run
out of resources, the user always has the option to kill applications
and use one thing at a time. On the other hand, if a single
application needs more than 50% of the SRAM and we don't allow it in
our heaps, the application will never work (unless, of course, the
user patches Mesa). So in this commit we go back to allowing
high-demanding applications to work by restoring the 50%-or-75% rule.

This commit is especially useful in systems with integrated graphics,
like LNL, where the option to upgrade RAM is not present.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28513>

src/intel/vulkan/anv_physical_device.c

@@ -1993,7 +1993,10 @@ anv_restrict_sys_heap_size(struct anv_physical_device *device,
          return kmd_reported_sram;
    }
 
-   return kmd_reported_sram / 2;
+   if (kmd_reported_sram <= 4ull * 1024ull * 1024ull * 1024ull)
+      return kmd_reported_sram / 2;
+   else
+      return kmd_reported_sram * 3 / 4;
 }
 
 static VkResult MUST_CHECK