anv: implement VK_EXT_global_priority extension

anv: implement VK_EXT_global_priority extension

v2: add ANV_CONTEXT_REALTIME_PRIORITY (Chris)
    use unreachable with unknown priority (Samuel)

v3: add stubs in gem_stubs.c (Emil)
    use priority defines from gen_defines.h

v4: cleanup, add anv_gem_set_context_param (Jason)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com> (v2)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v3)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>