hwcontext_vaapi: Make default DRM device selection more helpful

hwcontext_vaapi: Make default DRM device selection more helpful

Iterate over available render devices and pick the first one which looks
usable.  Adds an option to specify the name of the kernel driver associated
with the desired device, so that it is possible to select a specific type
of device in a multiple-device system without knowing the card numbering.

For example: -init_hw_device vaapi:,kernel_driver=amdgpu will select only
devices using the "amdgpu" driver (as used with recent AMD graphics cards).

Kernel driver selection requires libdrm to work.