d3d12: Add mechanism for D3D12 Adapter Selection

This add a simple mechanism to select which GPU adapter the d3d12
driver should be using. A new environment variable is introduced.

MESA_D3D12_DEFAULT_ADAPTER_NAME

This represent a substring to search for in the GPU descrition,
for example "NVIDIA" or "INTEL", or "NVIDIA GeForce RTX 3090",
etc...

GPU are searched in order and the first one to include the substring
becomes a match. If no match is found, we default to the first
enumerated GPU.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10710>

docs/drivers/d3d12.rst

@@ -47,3 +47,15 @@ environment variables:
 
 .. _debug layer: https://docs.microsoft.com/en-us/windows/win32/direct3d12/understanding-the-d3d12-debug-layer
 .. _GPU validator: https://docs.microsoft.com/en-us/windows/win32/direct3d12/using-d3d12-debug-layer-gpu-based-validation
+
+Utilities
+---------
+
+Environment variables that control the behavior of the D3D12 driver.
+
+.. envvar:: MESA_D3D12_DEFAULT_ADAPTER_NAME <string> ("")
+
+Specifies a substring to search for when chosing a default adapter to run on. 
+The first adapter matching the substring is chosen. The substring is not
+case sensitive.
+