device_select_layer: apply DRI_PRIME even if default device is > 1 to match opengl...

device_select_layer: apply DRI_PRIME even if default device is > 1 to match opengl behavior

This fixes DRI_PRIME=1 doing nothing if the GPU order is [dGPU, dGPU,
iGPU, cpu] (or more GPUs than that) and the iGPU is the default GPU.

Fixes: afa1fba1 ("vulkan/device_select: don't pick a cpu driver as the default")

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19101>