amdgpu: implement context priority for amdgpu_cs_ctx_create2 v3

Add a new context creation function that allows specifying the context
priority.

A high priority context has the potential of starving lower priority
contexts. The current kernel driver implementation allows only apps
that hold CAP_SYS_NICE or DRM_MASTER to acquire a priority above
AMDGPU_CTX_PRIORITY_NORMAL.

v2: corresponding changes for kernel patch v2
v3: Fixed 'make check' symbol error

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>