anv: do not open random render node(s)

anv: do not open random render node(s)

drmGetDevices2() provides us with enough flexibility to build heuristics
upon. Opening a random node on the other hand will wake up the device,
regardless if it's the one we're interested or not.

v2: Rebase, explicitly require/check for libdrm
v3: Return VK_ERROR_INCOMPATIBLE_DRIVER for no devices (Ilia)
v4: Rebase

Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)
Tested-by: Mike Lothian <mike@fireburn.co.uk>