nouveau: import and install a selection of nvif headers from the kernel

This commit also modifies the install path of the main libdrm_nouveau
header to be under a nouveau/ subdirectory.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>