gallium: implement get_device_vendor() for existing drivers

gallium: implement get_device_vendor() for existing drivers

The only hackish ones are llvmpipe and softpipe, which currently return
the same string as for get_vendor(), while ideally they should return
the CPU vendor.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>