drv: add drv_preload

It can be used to preload a backend, currently only used to preload
amdgpu.

BUG=b:269664560
TEST="grep radeonsi /proc/$(pidof zygote)/maps" on grunt

Change-Id: I30d9f701f1da3bc243a9c81326a1ba0c5e0562de
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/4262034
Commit-Queue: Chia-I Wu <[email protected]>
Reviewed-by: Yiwei Zhang <[email protected]>
Tested-by: Chia-I Wu <[email protected]>
4 files changed